JavaScript

16 sections · 146 entries

Variables & Constants

let Declaration

syntax

let variableName = value;

example

let score = 0;
score = 10;
console.log(score);

output

Note Block-scoped. Cannot be re-declared in the same scope. Preferred for values that change.

declare variablemutable variableblock scope variablelet vs var

const Declaration

syntax

const variableName = value;

example

const API_URL = "https://api.example.com";
const cart = ["apple"];
cart.push("banana"); // allowed
console.log(cart);

output

["apple", "banana"]

Note Block-scoped. Must be initialized at declaration. The binding is immutable, but object/array contents can still change.

constant variableimmutable bindingconst object mutationdeclare constant

var Declaration (Legacy)

syntax

var variableName = value;

example

function demo() {
  if (true) {
    var leaked = "visible outside block";
  }
  console.log(leaked);
}
demo();

output

"visible outside block"

Note Function-scoped, not block-scoped. Hoisted to the top of the function. Avoid in modern code -- use let or const instead.

var keywordfunction scopehoistinglegacy variablevar vs let

Array Destructuring

syntax

const [a, b, ...rest] = array;

example

const rgb = [30, 120, 255];
const [red, green, blue] = rgb;
console.log(green);

const [first, , third] = ["a", "b", "c"];
console.log(first, third);

output

120
"a" "c"

Note Use commas to skip elements. Works with any iterable, not just arrays.

unpack arrayarray destructuringextract array elementsswap variables

Object Destructuring

syntax

const { key1, key2: alias } = object;

example

const user = { name: "Lina", role: "admin", id: 42 };
const { name, role, id: userId } = user;
console.log(name, userId);

output

"Lina" 42

Note Use colon to rename. Combine with defaults: const { theme = "light" } = settings;

unpack objectobject destructuringrename destructured propertyextract object values

Nested Destructuring

syntax

const { outer: { inner } } = obj;

example

const response = {
  data: { user: { name: "Kai", scores: [95, 88] } }
};
const { data: { user: { name, scores: [latest] } } } = response;
console.log(name, latest);

output

"Kai" 95

Note Deeply nested destructuring can hurt readability. Consider extracting in steps for complex structures.

deep destructuringnested object destructuringextract nested value

Spread Operator

syntax

const merged = [...arr1, ...arr2];
const merged = { ...obj1, ...obj2 };

example

const defaults = { theme: "light", lang: "en" };
const prefs = { theme: "dark", fontSize: 16 };
const config = { ...defaults, ...prefs };
console.log(config);

output

{ theme: "dark", lang: "en", fontSize: 16 }

Note Later properties overwrite earlier ones. Only performs a shallow copy -- nested objects are still shared by reference.

spread arrayspread objectmerge objectscopy arrayshallow copy

Rest Parameters

syntax

function fn(...args) {}

example

function sum(...numbers) {
  return numbers.reduce((total, n) => total + n, 0);
}
console.log(sum(5, 10, 15));

output

Note Rest must be the last parameter. Unlike the old arguments object, rest gives you a real Array with all array methods.

rest parametersvariable argumentscollect argumentsvariadic function

Destructuring with Defaults

syntax

const { key = defaultValue } = obj;
const [a = defaultValue] = arr;

example

const { host = "localhost", port = 3000 } = { port: 8080 };
console.log(host, port);

output

"localhost" 8080

Note Defaults only apply when the value is undefined, not when it is null or other falsy values.

default values destructuringfallback valuedestructuring default

Variable Swapping

syntax

[a, b] = [b, a];

example

let x = "hello";
let y = "world";
[x, y] = [y, x];
console.log(x, y);

output

"world" "hello"

Note No temporary variable needed. Works with any number of variables: [a, b, c] = [c, a, b];

swap variablesexchange valuesswap without temp

Data Types

Primitive Types

syntax

string | number | boolean | undefined | null | symbol | bigint

example

const name = "Mira";       // string
const age = 28;            // number
const active = true;       // boolean
const missing = undefined; // undefined
const empty = null;        // null
const id = Symbol("id");   // symbol
const big = 900719925474099267n; // bigint

Note Primitives are immutable and compared by value. There are 7 primitive types in total.

primitive typesdata typesjavascript typesvalue types

typeof Operator

syntax

typeof value

example

console.log(typeof "hello");    // "string"
console.log(typeof 42);         // "number"
console.log(typeof true);       // "boolean"
console.log(typeof undefined);  // "undefined"
console.log(typeof null);       // "object" (!)  
console.log(typeof []);         // "object"
console.log(typeof (() => {})); // "function"

Note typeof null === "object" is a historic bug that will never be fixed. Use value === null for null checks. Arrays report as "object" -- use Array.isArray() instead.

check typetypeoftype checkingdetermine type

Type Coercion

syntax

implicit: value + "" | +value | !!value
explicit: String(v) | Number(v) | Boolean(v)

example

console.log("5" + 3);    // "53" (string concat)
console.log("5" - 3);    // 2   (numeric)
console.log(+"42");       // 42  (to number)
console.log(!!"hello");   // true (to boolean)
console.log(Number(""));  // 0
console.log(Number("ab")); // NaN

Note The + operator with a string always coerces to string. Prefer explicit conversion (Number(), String()) to avoid surprises.

type coercionimplicit conversionconvert typestring to numbertruthy falsy

Nullish Coalescing (??)

syntax

value ?? fallback

example

const inputCount = 0;
console.log(inputCount || 10);  // 10 (wrong!)
console.log(inputCount ?? 10);  // 0  (correct)

const label = null;
console.log(label ?? "Untitled");

output

10
0
"Untitled"

Note Only triggers on null/undefined, unlike || which triggers on all falsy values (0, "", false, NaN). Use ?? when 0 or empty string are valid values.

nullish coalescingnull fallbackdefault value null?? operatornull or undefined check

Optional Chaining (?.)

syntax

obj?.prop
obj?.[expr]
obj?.method()

example

const user = { profile: { avatar: "pic.png" } };
console.log(user?.profile?.avatar);  // "pic.png"
console.log(user?.settings?.theme);  // undefined
console.log(user?.getName?.());      // undefined (no error)

Note Short-circuits to undefined when a link in the chain is null/undefined. Combine with ?? for defaults: user?.name ?? "Guest"

optional chainingsafe property accessnull safe?. operatoravoid cannot read property

Nullish Assignment (??=)

syntax

variable ??= value;

example

let config = { timeout: null, retries: 3 };
config.timeout ??= 5000;
config.retries ??= 10;
console.log(config);

output

{ timeout: 5000, retries: 3 }

Note Only assigns when the left side is null or undefined. Also available: ||= (assigns on falsy) and &&= (assigns on truthy).

nullish assignment??= operatorassign if nulllogical assignment

Symbols

syntax

const sym = Symbol(description);

example

const STATUS = Symbol("status");
const order = { [STATUS]: "shipped", id: 101 };

console.log(order[STATUS]);       // "shipped"
console.log(Object.keys(order));  // ["id"]

Note Symbols are unique and hidden from normal enumeration. Use Symbol.for("key") to create shared/global symbols across modules.

symbolunique keyhidden propertysymbol type

Falsy Values

syntax

false | 0 | -0 | 0n | "" | null | undefined | NaN

example

const values = [false, 0, -0, 0n, "", null, undefined, NaN];
const truthyOnes = values.filter(Boolean);
console.log(truthyOnes.length);

output

Note Everything else is truthy, including empty objects {}, empty arrays [], and the string "false". This trips up many developers.

falsy valuestruthy falsyboolean conversionwhich values are false

Equality: == vs === vs Object.is

syntax

a === b   // strict (no coercion)
a == b    // loose (with coercion)
Object.is(a, b)  // same-value equality

example

console.log(0 == false);         // true
console.log(0 === false);        // false
console.log(NaN === NaN);        // false
console.log(Object.is(NaN, NaN)); // true
console.log(Object.is(0, -0));    // false

Note Always use === for comparisons. Object.is() handles edge cases like NaN and -0 that even === gets wrong.

equality== vs ===strict equalitycompare valuesObject.is

Strings

Template Literals

syntax

`text ${expression} text`

example

const item = "coffee";
const price = 4.5;
console.log(`One ${item} costs $${price.toFixed(2)}.`);

const multiline = `Line one
Line two
Line three`;
console.log(multiline);

output

"One coffee costs $4.50."
"Line one\nLine two\nLine three"

Note Backtick strings support embedded expressions and multiline content without escape characters.

template literalstring interpolationbacktick stringmultiline stringembed expression in string

Tagged Templates

syntax

tagFunction`text ${expr} text`

example

function highlight(strings, ...values) {
  return strings.reduce((result, str, i) => {
    return result + str + (values[i] !== undefined ? `[${values[i]}]` : "");
  }, "");
}
const user = "Kai";
console.log(highlight`Welcome, ${user}! You have ${3} alerts.`);

output

"Welcome, [Kai]! You have [3] alerts."

Note Tagged templates let you process template literal parts before producing the final string. Used in libraries like styled-components and GraphQL.

tagged templatetemplate tag functionprocess template literal

String Searching

syntax

str.includes(search, startIndex)
str.startsWith(search)
str.endsWith(search)

example

const msg = "Order #1234 shipped";
console.log(msg.includes("shipped"));   // true
console.log(msg.startsWith("Order"));   // true
console.log(msg.endsWith("shipped"));   // true
console.log(msg.indexOf("#"));          // 6

Note All search methods are case-sensitive. For case-insensitive checks, lowercase both sides first: str.toLowerCase().includes(term.toLowerCase()).

string containscheck if string includesstarts withends withfind in string

Extracting Substrings

syntax

str.slice(start, end)
str.substring(start, end)

example

const path = "/users/profile/avatar.png";
console.log(path.slice(1));          // "users/profile/avatar.png"
console.log(path.slice(-10));        // "avatar.png"
console.log(path.slice(7, 14));      // "profile"

Note slice() supports negative indices (counts from end). substring() does not. Prefer slice() -- it is more predictable.

substringslice stringextract part of stringget characters from string

Replacing Text

syntax

str.replace(search, replacement)
str.replaceAll(search, replacement)

example

const template = "Hello {name}, welcome to {place}!";
const filled = template
  .replace("{name}", "Ava")
  .replace("{place}", "Berlin");
console.log(filled);

const csv = "a,b,c,d";
console.log(csv.replaceAll(",", " | "));

output

"Hello Ava, welcome to Berlin!"
"a | b | c | d"

Note replace() only swaps the first match unless you use a regex with the /g flag. replaceAll() replaces every occurrence.

replace stringstring replacereplace all occurrencesfind and replace

Split and Join

syntax

str.split(separator, limit)
arr.join(separator)

example

const tags = "js,react,node";
const tagArray = tags.split(",");
console.log(tagArray);
console.log(tagArray.join(" + "));

output

["js", "react", "node"]
"js + react + node"

Note split("") splits into individual characters. split() with no arguments returns the entire string in a single-element array.

split stringjoin array to stringstring to arrayseparate by delimiter

Trimming Whitespace

syntax

str.trim()
str.trimStart()
str.trimEnd()

example

const input = "  hello@example.com  ";
console.log(input.trim());       // "hello@example.com"
console.log(input.trimStart());  // "hello@example.com  "
console.log(input.trimEnd());    // "  hello@example.com"

Note Essential for cleaning user input. Only removes whitespace characters (spaces, tabs, newlines), not other invisible characters.

trim whitespaceremove spacestrim stringclean input

Padding Strings

syntax

str.padStart(targetLength, padChar)
str.padEnd(targetLength, padChar)

example

const orderNum = "42";
console.log(orderNum.padStart(6, "0"));  // "000042"

const label = "Price";
console.log(label.padEnd(12, "."));      // "Price......."

Note Useful for formatting IDs, aligning columns in console output, or zero-padding numbers.

pad stringpadStartpadEndzero padformat string length

Case Conversion

syntax

str.toUpperCase()
str.toLowerCase()
str.toLocaleUpperCase(locale)

example

const status = "Pending";
console.log(status.toUpperCase());  // "PENDING"
console.log(status.toLowerCase());  // "pending"

// Locale-aware (e.g., Turkish "i" -> "I" with dot)
console.log("istanbul".toLocaleUpperCase("tr"));

Note Always use locale-aware methods when dealing with internationalized text. The Turkish i is a classic bug source.

uppercaselowercaseconvert casecapitalize string

Repeat and Character Access

syntax

str.repeat(count)
str.at(index)

example

const border = "=".repeat(30);
console.log(border);

const word = "JavaScript";
console.log(word.at(0));   // "J"
console.log(word.at(-1));  // "t"

Note at() supports negative indices to count from the end. Bracket notation str[-1] returns undefined, not the last character.

repeat stringget last characterstring atcharacter at index

Numbers & Math

Parsing Numbers

syntax

Number(value)
parseInt(string, radix)
parseFloat(string)

example

console.log(Number("42"));        // 42
console.log(Number("3.14abc"));   // NaN
console.log(parseInt("3.14abc")); // 3
console.log(parseFloat("3.14abc")); // 3.14
console.log(parseInt("ff", 16));  // 255

Note Number() is stricter -- rejects partially numeric strings. parseInt() stops at the first non-numeric character. Always pass the radix to parseInt() to avoid octal surprises.

parse numberstring to numberparseIntparseFloatconvert to number

Number Checking

syntax

Number.isNaN(value)
Number.isFinite(value)
Number.isInteger(value)
Number.isSafeInteger(value)

example

console.log(Number.isNaN(NaN));          // true
console.log(Number.isNaN("hello"));      // false
console.log(isNaN("hello"));             // true (!) 
console.log(Number.isInteger(4.0));       // true
console.log(Number.isSafeInteger(2**53)); // false

Note Always use Number.isNaN() over the global isNaN(). The global version coerces its argument to a number first, giving misleading results.

check NaNisNaNis numbercheck integervalidate number

Rounding

syntax

Math.round(n) | Math.floor(n) | Math.ceil(n) | Math.trunc(n)

example

const price = 19.872;
console.log(Math.round(price));  // 20
console.log(Math.floor(price));  // 19
console.log(Math.ceil(price));   // 20
console.log(Math.trunc(price));  // 19
console.log(Math.trunc(-3.7));   // -3 (not -4)

Note trunc() simply removes decimals (towards zero). floor() rounds towards negative infinity. They differ for negative numbers: floor(-3.2) = -4 but trunc(-3.2) = -3.

round numberfloorceiltruncateround downround up

Formatting Decimals

syntax

num.toFixed(digits)
num.toPrecision(precision)

example

const total = 29.5;
console.log(total.toFixed(2));       // "29.50"
console.log((0.1 + 0.2).toFixed(2)); // "0.30"

const big = 123456.789;
console.log(big.toPrecision(6));     // "123457"

Note toFixed() returns a STRING, not a number. Wrap in Number() or use + to convert back: +total.toFixed(2)

format decimaltoFixeddecimal placesformat currencynumber to string

Random Numbers

syntax

Math.random()  // [0, 1)

example

// Random float between 0 and 1
console.log(Math.random());

// Random integer from min to max (inclusive)
function randomInt(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}
console.log(randomInt(1, 100));

Note Math.random() is NOT cryptographically secure. For tokens or passwords, use crypto.getRandomValues() or crypto.randomUUID().

random numbergenerate randomrandom integerMath.random

Min, Max, and Clamping

syntax

Math.min(...values)
Math.max(...values)

example

const scores = [88, 45, 99, 72, 61];
console.log(Math.min(...scores)); // 45
console.log(Math.max(...scores)); // 99

// Clamp a value between bounds
function clamp(value, min, max) {
  return Math.min(Math.max(value, min), max);
}
console.log(clamp(150, 0, 100)); // 100

Note For very large arrays (100k+ elements), spread can cause a stack overflow. Use a reduce loop instead.

min valuemax valuefind minimumfind maximumclamp number

BigInt

syntax

const big = 123n;
const big = BigInt(value);

example

const huge = 9007199254740993n;
console.log(huge + 1n);   // 9007199254740994n
console.log(huge * 2n);   // 18014398509481986n

// Convert between types
console.log(Number(100n)); // 100
console.log(BigInt(42));   // 42n

Note Cannot mix BigInt and Number in arithmetic (throws TypeError). Convert one type first. No Math methods work with BigInt.

BigIntlarge numbersarbitrary precisionbig integer

Powers and Roots

syntax

base ** exponent
Math.sqrt(n)
Math.cbrt(n)
Math.pow(base, exp)

example

console.log(2 ** 10);         // 1024
console.log(Math.sqrt(144));  // 12
console.log(Math.cbrt(27));   // 3
console.log(Math.abs(-42));   // 42

Note The ** operator is cleaner than Math.pow() and works with BigInt too: 2n ** 64n.

exponentpowersquare rootabsolute valueMath.pow

Locale Number Formatting

syntax

new Intl.NumberFormat(locale, options).format(n)

example

const price = 1234567.89;
console.log(new Intl.NumberFormat("en-US", {
  style: "currency", currency: "USD"
}).format(price));
// "$1,234,567.89"

console.log(new Intl.NumberFormat("de-DE").format(price));
// "1.234.567,89"

Note Intl.NumberFormat handles thousands separators, currency symbols, percentages, and units correctly for any locale.

format numbercurrency formatlocale numberthousands separatorIntl.NumberFormat

Arrays

Creating Arrays

syntax

const arr = [a, b, c];
const arr = Array.from(iterable);
const arr = Array.of(elements);

example

const fruits = ["apple", "banana", "cherry"];
const range = Array.from({ length: 5 }, (_, i) => i + 1);
console.log(range);  // [1, 2, 3, 4, 5]

const filled = new Array(3).fill(0);
console.log(filled); // [0, 0, 0]

Note Array(5) creates 5 empty slots, not [5]. Use Array.of(5) for a single-element array. Be careful with fill() on objects -- all slots share the same reference.

create arraynew arrayArray.fromgenerate arrayfill array

Accessing Elements

syntax

arr[index]
arr.at(index)

example

const colors = ["red", "green", "blue", "yellow"];
console.log(colors[0]);     // "red"
console.log(colors.at(-1));  // "yellow"
console.log(colors.at(-2));  // "blue"

Note at() supports negative indexing. Bracket notation with negatives (arr[-1]) returns undefined because it looks for a property named "-1".

access array elementget last elementarray atarray index

Adding and Removing Elements

syntax

arr.push(item)    // end
arr.pop()         // end
arr.unshift(item) // start
arr.shift()       // start
arr.splice(index, deleteCount, ...items)

example

const tasks = ["code", "test"];
tasks.push("deploy");       // ["code", "test", "deploy"]
tasks.unshift("plan");      // ["plan", "code", "test", "deploy"]
const removed = tasks.pop(); // "deploy"

// Insert at index 2
tasks.splice(2, 0, "review");
console.log(tasks); // ["plan", "code", "review", "test"]

Note push/pop are O(1). unshift/shift are O(n) because all indices must be renumbered. splice() mutates the original array.

add to arrayremove from arraypushpopspliceinsert into array

map()

syntax

arr.map(callback(element, index, array))

example

const prices = [10, 25, 50];
const withTax = prices.map(p => +(p * 1.08).toFixed(2));
console.log(withTax);

output

[10.8, 27, 54]

Note map() returns a new array of the same length. If you do not need the return value, use forEach() instead.

transform arraymap arrayapply function to eachconvert array elements

filter()

syntax

arr.filter(callback(element, index, array))

example

const products = [
  { name: "Shirt", price: 25 },
  { name: "Jacket", price: 120 },
  { name: "Cap", price: 15 },
];
const affordable = products.filter(p => p.price < 50);
console.log(affordable.map(p => p.name));

output

["Shirt", "Cap"]

Note Returns a new array with elements that pass the test. The callback must return a truthy/falsy value.

filter arrayselect elementsremove from array by conditionarray where

reduce()

syntax

arr.reduce(callback(accumulator, current, index, array), initialValue)

example

const items = [
  { name: "Book", price: 12 },
  { name: "Pen", price: 3 },
  { name: "Bag", price: 45 },
];
const total = items.reduce((sum, item) => sum + item.price, 0);
console.log(total);

output

Note Always provide an initial value (second argument). Without it, reduce uses the first element and can throw on empty arrays.

reduce arraysum arrayaccumulateaggregate arrayfold

find() and findIndex()

syntax

arr.find(callback)
arr.findIndex(callback)
arr.findLast(callback)
arr.findLastIndex(callback)

example

const users = [
  { id: 1, name: "Ava" },
  { id: 2, name: "Bo" },
  { id: 3, name: "Ava" },
];
console.log(users.find(u => u.name === "Ava"));     // { id: 1, name: "Ava" }
console.log(users.findLast(u => u.name === "Ava")); // { id: 3, name: "Ava" }
console.log(users.findIndex(u => u.id === 2));       // 1

Note find() returns the first match or undefined. findLast() (ES2023) searches from the end. Both return the element, not a boolean.

find in arraysearch arrayfindIndexfindLastlocate element

Sorting

syntax

arr.sort(compareFn)
arr.toSorted(compareFn)

example

const nums = [40, 1, 5, 200];
// WRONG: default sort is lexicographic
console.log([...nums].sort());             // [1, 200, 40, 5]
// CORRECT: numeric sort
console.log([...nums].sort((a, b) => a - b)); // [1, 5, 40, 200]

// Non-mutating sort (ES2023)
const sorted = nums.toSorted((a, b) => b - a);
console.log(sorted); // [200, 40, 5, 1]
console.log(nums);   // [40, 1, 5, 200] (unchanged)

Note sort() MUTATES the array and defaults to string comparison. Always pass a compare function for numbers. Use toSorted() for a non-mutating alternative.

sort arrayarray sortsort numberstoSortednon-mutating sort

flat() and flatMap()

syntax

arr.flat(depth)
arr.flatMap(callback)

example

const nested = [[1, 2], [3, [4, 5]]];
console.log(nested.flat());    // [1, 2, 3, [4, 5]]
console.log(nested.flat(2));   // [1, 2, 3, 4, 5]

const sentences = ["hello world", "good morning"];
const words = sentences.flatMap(s => s.split(" "));
console.log(words); // ["hello", "world", "good", "morning"]

Note flatMap() is equivalent to map() followed by flat(1) but more efficient. Use flat(Infinity) to flatten any depth.

flatten arrayflatflatMapnested arrayunwrap array

includes(), some(), every()

syntax

arr.includes(value)
arr.some(callback)
arr.every(callback)

example

const perms = ["read", "write", "delete"];
console.log(perms.includes("write"));  // true

const ages = [22, 17, 30, 15];
console.log(ages.some(a => a < 18));   // true
console.log(ages.every(a => a >= 18)); // false

Note includes() uses strict equality (===) and works with NaN. some() short-circuits on the first true; every() short-circuits on the first false.

array containscheck if in arrayincludessomeeveryany match

Non-Mutating Alternatives (ES2023)

syntax

arr.toSorted(fn)
arr.toReversed()
arr.toSpliced(start, deleteCount, ...items)
arr.with(index, value)

example

const original = [3, 1, 4, 1, 5];

const reversed = original.toReversed();
console.log(reversed);  // [5, 1, 4, 1, 3]

const replaced = original.with(2, 99);
console.log(replaced);  // [3, 1, 99, 1, 5]

console.log(original);  // [3, 1, 4, 1, 5] (untouched)

Note These return new arrays, leaving the original unchanged. Ideal for immutable state patterns (React, Redux, etc).

immutable arraytoSortedtoReversedtoSplicedarray withnon-mutating

Deep Cloning Arrays

syntax

structuredClone(value)

example

const original = [
  { name: "Ava", scores: [90, 85] },
  { name: "Leo", scores: [78, 92] },
];
const clone = structuredClone(original);
clone[0].scores.push(100);

console.log(original[0].scores); // [90, 85] (not affected)
console.log(clone[0].scores);    // [90, 85, 100]

Note structuredClone handles nested objects, arrays, Maps, Sets, Dates, RegExps, and more. Does NOT clone functions, DOM nodes, or prototypes.

deep clone arraycopy nested arraystructuredClonedeep copy

Objects

Creating Objects

syntax

const obj = { key: value };
const obj = Object.create(proto);

example

const user = {
  name: "Kai",
  age: 30,
  greet() {
    return `Hi, I'm ${this.name}`;
  }
};
console.log(user.greet());

output

"Hi, I'm Kai"

Note Method shorthand greet() {} is preferred over greet: function() {}. Arrow functions should not be used as methods because they do not bind their own this.

create objectobject literaldefine objectobject with methods

Accessing Properties

syntax

obj.property
obj["property"]
obj[variable]

example

const config = { "max-retries": 3, timeout: 5000 };
console.log(config.timeout);          // 5000
console.log(config["max-retries"]);   // 3

const key = "timeout";
console.log(config[key]);             // 5000

Note Use bracket notation for dynamic keys or keys with special characters. Dot notation is cleaner for static, valid-identifier keys.

access object propertydot notationbracket notationdynamic property

Property Shorthand and Computed Keys

syntax

const obj = { name, [expression]: value };

example

const name = "Mira";
const role = "admin";
const user = { name, role };
console.log(user); // { name: "Mira", role: "admin" }

const field = "email";
const form = { [field]: "mira@example.com" };
console.log(form); // { email: "mira@example.com" }

Note Shorthand works when the variable name matches the desired key name. Computed keys are evaluated at runtime.

shorthand propertycomputed property keydynamic keyvariable as key

Object Destructuring in Function Parameters

syntax

function fn({ key1, key2 = default }) {}

example

function createUser({ name, role = "viewer", active = true }) {
  return { name, role, active, createdAt: Date.now() };
}

const admin = createUser({ name: "Bo", role: "admin" });
console.log(admin);

output

{ name: "Bo", role: "admin", active: true, createdAt: ... }

Note Destructuring in parameters makes function signatures self-documenting. Add = {} as a default to allow calling with no arguments: function fn({ x } = {}).

destructure function paramsnamed parametersoptions object pattern

Object.keys / values / entries

syntax

Object.keys(obj)
Object.values(obj)
Object.entries(obj)

example

const inventory = { apples: 5, bananas: 12, cherries: 30 };

console.log(Object.keys(inventory));    // ["apples", "bananas", "cherries"]
console.log(Object.values(inventory));  // [5, 12, 30]

for (const [fruit, count] of Object.entries(inventory)) {
  console.log(`${fruit}: ${count}`);
}

Note These only return own enumerable string-keyed properties. Symbol keys and inherited properties are excluded.

iterate objectobject keysobject valuesloop over objectobject to array

Merging Objects

syntax

Object.assign(target, ...sources)
{ ...obj1, ...obj2 }

example

const defaults = { theme: "light", lang: "en", debug: false };
const userPrefs = { theme: "dark", debug: true };

const settings = { ...defaults, ...userPrefs };
console.log(settings);

output

{ theme: "dark", lang: "en", debug: true }

Note Both approaches are shallow. Later sources overwrite earlier ones. Use structuredClone for deep merging or a library for recursive merge.

merge objectscombine objectsObject.assignspread mergeextend object

Freezing and Sealing Objects

syntax

Object.freeze(obj)
Object.seal(obj)
Object.isFrozen(obj)

example

const API = Object.freeze({
  BASE_URL: "https://api.example.com",
  VERSION: 2,
});
API.VERSION = 3;            // silently fails (throws in strict mode)
console.log(API.VERSION);   // 2

Note freeze() prevents all changes. seal() allows modifying existing properties but not adding/removing. Both are shallow -- nested objects remain mutable.

freeze objectimmutable objectprevent modificationObject.freezeObject.seal

Checking Property Existence

syntax

Object.hasOwn(obj, prop)
"prop" in obj

example

const user = { name: "Lina", age: 28 };

console.log(Object.hasOwn(user, "name")); // true
console.log(Object.hasOwn(user, "email")); // false
console.log("toString" in user);           // true (inherited)
console.log(Object.hasOwn(user, "toString")); // false

Note Object.hasOwn() (ES2022) replaces obj.hasOwnProperty(). It works even if the object was created with Object.create(null).

check property existshasOwnPropertyObject.hasOwnproperty checkin operator

Object.fromEntries()

syntax

Object.fromEntries(iterable)

example

const params = new URLSearchParams("page=2&sort=name&order=asc");
const queryObj = Object.fromEntries(params);
console.log(queryObj);

// Transform values
const prices = { shirt: 25, jacket: 120, cap: 15 };
const discounted = Object.fromEntries(
  Object.entries(prices).map(([item, price]) => [item, price * 0.9])
);
console.log(discounted);

output

{ page: "2", sort: "name", order: "asc" }
{ shirt: 22.5, jacket: 108, cap: 13.5 }

Note The inverse of Object.entries(). Takes any iterable of [key, value] pairs including Maps.

entries to objectObject.fromEntriesarray to objectmap to object

Deep Cloning Objects

syntax

structuredClone(obj)

example

const original = {
  user: { name: "Kai", tags: ["admin", "editor"] },
  updatedAt: new Date(),
};
const clone = structuredClone(original);
clone.user.tags.push("viewer");

console.log(original.user.tags); // ["admin", "editor"]

Note Handles circular references, Dates, Maps, Sets, ArrayBuffers, and more. Does NOT clone functions, Errors, or DOM nodes.

deep clone objectcopy nested objectstructuredClonedeep copy object

Functions

Function Declaration

syntax

function name(params) { ... }

example

function calculateTip(bill, tipPercent = 18) {
  return +(bill * tipPercent / 100).toFixed(2);
}
console.log(calculateTip(85));     // 15.3
console.log(calculateTip(85, 20)); // 17

Note Declarations are hoisted -- they can be called before they appear in code. This is the only function form that hoists.

function declarationdefine functioncreate functionhoisted function

Arrow Functions

syntax

const fn = (params) => expression;
const fn = (params) => { ... };

example

const double = n => n * 2;
console.log(double(7)); // 14

const greet = (name, greeting = "Hello") => `${greeting}, ${name}!`;
console.log(greet("Ava")); // "Hello, Ava!"

// Return an object literal (wrap in parentheses)
const makeUser = (name, id) => ({ name, id });
console.log(makeUser("Bo", 1));

Note Arrow functions have no own this, arguments, or super. They inherit this from the surrounding scope, making them unsuitable as object methods or constructors.

arrow functionlambdashort functionfat arrow=> syntax

Default Parameters

syntax

function fn(param = defaultValue) {}

example

function fetchData(url, options = {}) {
  const { method = "GET", timeout = 3000 } = options;
  console.log(`${method} ${url} (timeout: ${timeout}ms)`);
}
fetchData("/api/users");
fetchData("/api/users", { method: "POST" });

output

"GET /api/users (timeout: 3000ms)"
"POST /api/users (timeout: 3000ms)"

Note Defaults are evaluated at call time, not definition time. Each call creates a fresh default value, so objects/arrays as defaults are safe.

default parameteroptional parameterdefault argumentparameter fallback

Rest Parameters in Functions

syntax

function fn(first, ...rest) {}

example

function logTagged(level, ...messages) {
  const timestamp = new Date().toISOString();
  console.log(`[${level}] ${timestamp}:`, ...messages);
}
logTagged("INFO", "Server started", "on port 3000");

output

[INFO] 2026-04-04T...: Server started on port 3000

Note Rest parameters collect remaining arguments into a real Array. Unlike arguments, they work in arrow functions too.

rest parametersvariable argumentsgather argumentsvariadic

Closures

syntax

function outer() {
  let state = value;
  return function inner() { /* access state */ };
}

example

function createCounter(initial = 0) {
  let count = initial;
  return {
    increment() { return ++count; },
    decrement() { return --count; },
    value()     { return count; },
  };
}
const counter = createCounter(10);
console.log(counter.increment()); // 11
console.log(counter.increment()); // 12
console.log(counter.value());     // 12

Note A closure is a function that retains access to its outer scope's variables even after the outer function has returned. Fundamental for data privacy and stateful functions.

closurefunction scopeprivate variableenclosed variablestateful function

IIFE (Immediately Invoked Function Expression)

syntax

(function() { ... })();
(() => { ... })();

example

const api = (() => {
  let requestCount = 0;
  return {
    fetch(url) {
      requestCount++;
      console.log(`Request #${requestCount}: ${url}`);
    },
    getCount() { return requestCount; }
  };
})();
api.fetch("/users");
console.log(api.getCount());

output

"Request #1: /users"
1

Note IIFEs were essential before modules for avoiding global scope pollution. Still useful for one-time initialization blocks.

IIFEimmediately invokedself executing functionmodule pattern

Generator Functions

syntax

function* name() { yield value; }

example

function* idGenerator(start = 1) {
  let id = start;
  while (true) {
    yield id++;
  }
}
const ids = idGenerator(100);
console.log(ids.next().value); // 100
console.log(ids.next().value); // 101
console.log(ids.next().value); // 102

Note Generators are lazy -- they produce values on demand. Execution pauses at each yield and resumes when next() is called.

generatoryieldlazy iterationfunction*iterator generator

Async Generator Functions

syntax

async function* name() { yield await value; }

example

async function* fetchPages(baseUrl, maxPages = 3) {
  for (let page = 1; page <= maxPages; page++) {
    const res = await fetch(`${baseUrl}?page=${page}`);
    yield await res.json();
  }
}

// Usage
for await (const pageData of fetchPages("/api/posts")) {
  console.log(`Got ${pageData.items.length} items`);
}

Note Consume with for-await-of. Ideal for paginated APIs, streaming data, or any sequence of async operations.

async generatorasync iteratorfor await ofpaginated fetchstreaming data

Higher-Order Functions

syntax

function fn(callback) { callback(); }
function fn() { return function() {}; }

example

function withLogging(fn) {
  return function(...args) {
    console.log(`Calling ${fn.name} with`, args);
    const result = fn(...args);
    console.log(`Result:`, result);
    return result;
  };
}

const add = (a, b) => a + b;
const loggedAdd = withLogging(add);
loggedAdd(3, 4);

output

"Calling add with [3, 4]"
"Result: 7"

Note Functions that accept or return other functions. The backbone of functional patterns like decorators, middleware, and composition.

higher order functionfunction as argumentreturn functioncallback patternwrapper function

Control Flow

if / else if / else

syntax

if (condition) { ... }
else if (condition) { ... }
else { ... }

example

function getDiscount(memberLevel) {
  if (memberLevel === "gold") {
    return 0.20;
  } else if (memberLevel === "silver") {
    return 0.10;
  } else {
    return 0;
  }
}
console.log(getDiscount("gold")); // 0.20

Note Conditions are coerced to boolean. Watch out for truthy/falsy gotchas: if (arr.length) works because 0 is falsy.

if elseconditionalbranchingif statement

Ternary Operator

syntax

condition ? valueIfTrue : valueIfFalse

example

const age = 20;
const category = age >= 18 ? "adult" : "minor";
console.log(category); // "adult"

// Nested (use sparingly)
const tier = age >= 65 ? "senior" : age >= 18 ? "adult" : "minor";

Note Great for simple inline conditions. Avoid deeply nested ternaries -- they quickly become unreadable. Use if/else for complex logic.

ternaryinline ifconditional expression? : operator

switch Statement

syntax

switch (expression) {
  case value: ... break;
  default: ...
}

example

function getStatusText(code) {
  switch (code) {
    case 200: return "OK";
    case 301: return "Moved Permanently";
    case 404: return "Not Found";
    case 500: return "Internal Server Error";
    default:  return `Unknown (${code})`;
  }
}
console.log(getStatusText(404));

output

"Not Found"

Note Uses strict equality (===). Forgetting break causes fall-through to the next case. Using return instead of break is a clean pattern in functions.

switch caseswitch statementmultiple conditionsselect case

for Loop

syntax

for (init; condition; update) { ... }

example

const items = ["apple", "banana", "cherry"];
for (let i = 0; i < items.length; i++) {
  console.log(`${i + 1}. ${items[i]}`);
}

output

"1. apple"
"2. banana"
"3. cherry"

Note Classic loop when you need the index. For simple iteration, prefer for...of. Cache .length in the initializer if the array is very large and not changing.

for looploop with indexiterate with counterclassic loop

for...of Loop

syntax

for (const element of iterable) { ... }

example

const scores = [88, 92, 75, 100];
let sum = 0;
for (const score of scores) {
  sum += score;
}
console.log(`Average: ${sum / scores.length}`);

// Works with strings too
for (const char of "hello") {
  process(char);
}

output

"Average: 88.75"

Note Works on any iterable: arrays, strings, Maps, Sets, generators. Does NOT work on plain objects -- use Object.entries() or for...in for those.

for of loopiterate arrayloop iterablefor of

for...in Loop

syntax

for (const key in object) { ... }

example

const theme = { primary: "#3498db", secondary: "#2ecc71", accent: "#e74c3c" };
for (const colorName in theme) {
  console.log(`${colorName}: ${theme[colorName]}`);
}

output

"primary: #3498db"
"secondary: #2ecc71"
"accent: #e74c3c"

Note Iterates over enumerable string properties, including inherited ones. Avoid for arrays (use for...of). Use Object.hasOwn() to filter inherited keys.

for in looploop object keysiterate propertiesfor in

while and do...while

syntax

while (condition) { ... }
do { ... } while (condition);

example

// Retry until success (with limit)
let attempts = 0;
let success = false;
while (!success && attempts < 5) {
  attempts++;
  success = Math.random() > 0.7;
}
console.log(`Succeeded after ${attempts} attempt(s): ${success}`);

// do...while always runs at least once
let input;
do {
  input = prompt("Enter a number > 10:");
} while (Number(input) <= 10);

Note do...while guarantees at least one iteration. Useful for retry logic and input validation loops.

while loopdo whileloop until conditionrepeat until

break and continue

syntax

break;     // exit loop
continue;  // skip to next iteration

example

const data = [3, -1, 7, 0, 5, -3, 9];
const positives = [];

for (const n of data) {
  if (n < 0) continue;  // skip negatives
  if (positives.length >= 3) break;  // stop after 3
  positives.push(n);
}
console.log(positives); // [3, 7, 5]

Note break exits the innermost loop. Use labeled breaks to exit outer loops: outer: for (...) { for (...) { break outer; } }

break loopcontinue loopskip iterationexit loop early

Labeled Statements

syntax

label: for (...) { break label; }

example

const matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9],
];

let found = null;
search: for (const row of matrix) {
  for (const cell of row) {
    if (cell === 5) {
      found = cell;
      break search;
    }
  }
}
console.log(found); // 5

Note Labels let you break/continue a specific outer loop from inside a nested loop. Rarely needed but invaluable for searching nested structures.

labeled statementbreak outer loopnested loop breaklabel break

Async Programming

Promise Basics

syntax

new Promise((resolve, reject) => { ... })
promise.then(onFulfilled, onRejected)
promise.catch(onRejected)
promise.finally(onFinally)

example

function delay(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

delay(1000)
  .then(() => console.log("1 second passed"))
  .catch(err => console.error(err))
  .finally(() => console.log("Done"));

Note A Promise is always in one of three states: pending, fulfilled, or rejected. Once settled, it cannot change state.

promisecreate promisethen catchasynchronouspromise basics

async / await

syntax

async function name() {
  const result = await promise;
}

example

async function loadUser(userId) {
  const response = await fetch(`/api/users/${userId}`);
  if (!response.ok) {
    throw new Error(`HTTP ${response.status}`);
  }
  const user = await response.json();
  return user;
}

// Usage
try {
  const user = await loadUser(42);
  console.log(user.name);
} catch (err) {
  console.error("Failed to load user:", err.message);
}

Note await pauses execution inside an async function until the promise settles. Top-level await works in ES modules. Always handle errors with try/catch or .catch().

async awaitawait promiseasync functionasynchronous function

Promise.all()

syntax

Promise.all([p1, p2, p3])

example

async function loadDashboard(userId) {
  const [profile, posts, notifications] = await Promise.all([
    fetch(`/api/users/${userId}`).then(r => r.json()),
    fetch(`/api/posts?author=${userId}`).then(r => r.json()),
    fetch(`/api/notifications/${userId}`).then(r => r.json()),
  ]);
  return { profile, posts, notifications };
}

Note Runs all promises in parallel and resolves when ALL succeed. Rejects immediately if ANY promise rejects -- other pending promises are not cancelled, just ignored.

parallel promisesPromise.allconcurrent requestswait for all

Promise.allSettled()

syntax

Promise.allSettled([p1, p2, p3])

example

const results = await Promise.allSettled([
  fetch("/api/service-a"),
  fetch("/api/service-b"),
  fetch("/api/service-c"),
]);

const successful = results.filter(r => r.status === "fulfilled");
const failed = results.filter(r => r.status === "rejected");
console.log(`${successful.length} succeeded, ${failed.length} failed`);

Note Never rejects. Each result is { status: "fulfilled", value } or { status: "rejected", reason }. Use when you want all results regardless of failures.

promise allSettledignore failuresall resultspartial success

Promise.race()

syntax

Promise.race([p1, p2, p3])

example

function fetchWithTimeout(url, ms = 5000) {
  const timeout = new Promise((_, reject) =>
    setTimeout(() => reject(new Error("Timeout")), ms)
  );
  return Promise.race([fetch(url), timeout]);
}

try {
  const response = await fetchWithTimeout("/api/data", 3000);
  console.log(await response.json());
} catch (err) {
  console.error(err.message); // "Timeout" or network error
}

Note Settles as soon as the first promise settles (fulfilled OR rejected). Common use: implementing timeouts.

promise racefirst to finishtimeout promisefastest promise

Promise.any()

syntax

Promise.any([p1, p2, p3])

example

async function fetchFromMirrors(path) {
  return Promise.any([
    fetch(`https://mirror1.example.com${path}`),
    fetch(`https://mirror2.example.com${path}`),
    fetch(`https://mirror3.example.com${path}`),
  ]);
}
// Returns the first successful response; ignores individual failures

Note Resolves with the first fulfilled promise. Only rejects if ALL promises reject (with an AggregateError). The opposite of Promise.race() for error handling.

promise anyfirst successfallback promiseAggregateError

Fetch API

syntax

const response = await fetch(url, options);

example

// GET request
const res = await fetch("/api/products");
const products = await res.json();

// POST request
const created = await fetch("/api/products", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ name: "Widget", price: 29.99 }),
});

if (!created.ok) throw new Error(`HTTP ${created.status}`);

Note fetch() does NOT reject on HTTP error status codes (404, 500). Always check response.ok or response.status. The body can only be consumed once.

fetchhttp requestGET requestPOST requestAPI call

AbortController

syntax

const controller = new AbortController();
fetch(url, { signal: controller.signal });
controller.abort();

example

async function searchWithCancel(query) {
  const controller = new AbortController();

  // Auto-cancel after 5 seconds
  const timeoutId = setTimeout(() => controller.abort(), 5000);

  try {
    const res = await fetch(`/api/search?q=${query}`, {
      signal: controller.signal,
    });
    clearTimeout(timeoutId);
    return await res.json();
  } catch (err) {
    if (err.name === "AbortError") {
      console.log("Request was cancelled");
    } else {
      throw err;
    }
  }
}

Note Essential for cancelling stale requests (e.g., in search-as-you-type). The AbortSignal can be shared across multiple fetch calls to cancel them all at once.

abort fetchcancel requestAbortControllerabort signalcancel promise

Async Error Handling Patterns

syntax

try { await ... } catch (err) { ... }

example

// Wrapper to avoid repetitive try/catch
async function safeAwait(promise) {
  try {
    const data = await promise;
    return [data, null];
  } catch (err) {
    return [null, err];
  }
}

const [user, error] = await safeAwait(fetch("/api/me").then(r => r.json()));
if (error) {
  console.error("Failed:", error.message);
} else {
  console.log("User:", user.name);
}

Note Unhandled promise rejections crash Node.js and show warnings in browsers. Always catch async errors. The tuple pattern [data, error] is popular for cleaner control flow.

async error handlingtry catch asynchandle rejectionerror tuple pattern

Classes & OOP

Class Declaration

syntax

class ClassName {
  constructor(params) { ... }
  method() { ... }
}

example

class Product {
  constructor(name, price) {
    this.name = name;
    this.price = price;
  }

  format() {
    return `${this.name}: $${this.price.toFixed(2)}`;
  }
}

const item = new Product("Notebook", 12.5);
console.log(item.format());

output

"Notebook: $12.50"

Note Classes are syntactic sugar over prototypes. They are NOT hoisted -- you must declare before use, unlike function declarations.

classcreate classconstructorclass methodOOP

Getters and Setters

syntax

get propertyName() { ... }
set propertyName(value) { ... }

example

class Temperature {
  #celsius;

  constructor(celsius) {
    this.#celsius = celsius;
  }

  get fahrenheit() {
    return this.#celsius * 9 / 5 + 32;
  }

  set fahrenheit(f) {
    this.#celsius = (f - 32) * 5 / 9;
  }

  get celsius() { return this.#celsius; }
}

const temp = new Temperature(100);
console.log(temp.fahrenheit); // 212
temp.fahrenheit = 32;
console.log(temp.celsius);    // 0

Note Getters and setters look like regular properties from the outside. They are ideal for computed values, validation, and encapsulation.

gettersetterget setcomputed propertyaccessor

Static Methods and Properties

syntax

class C {
  static method() { ... }
  static property = value;
}

example

class MathUtils {
  static PI = 3.14159265;

  static clamp(value, min, max) {
    return Math.min(Math.max(value, min), max);
  }

  static lerp(a, b, t) {
    return a + (b - a) * t;
  }
}

console.log(MathUtils.clamp(150, 0, 100)); // 100
console.log(MathUtils.lerp(0, 50, 0.5));  // 25

Note Static members belong to the class itself, not instances. Access them via ClassName.method(), not instance.method().

static methodstatic propertyclass methodutility class

Inheritance (extends)

syntax

class Child extends Parent {
  constructor(params) {
    super(parentParams);
  }
}

example

class Shape {
  constructor(color) {
    this.color = color;
  }
  describe() {
    return `A ${this.color} shape`;
  }
}

class Circle extends Shape {
  constructor(color, radius) {
    super(color);
    this.radius = radius;
  }
  get area() {
    return Math.PI * this.radius ** 2;
  }
  describe() {
    return `A ${this.color} circle (r=${this.radius})`;
  }
}

const c = new Circle("blue", 5);
console.log(c.describe());
console.log(c.area.toFixed(2));

output

"A blue circle (r=5)"
"78.54"

Note super() must be called in the child constructor before accessing this. Methods can be overridden. Use super.method() to call the parent version.

class inheritanceextendssupersubclassoverride method

Private Fields and Methods

syntax

class C {
  #privateField = value;
  #privateMethod() { ... }
}

example

class BankAccount {
  #balance = 0;

  constructor(initial) {
    this.#balance = initial;
  }

  deposit(amount) {
    this.#validateAmount(amount);
    this.#balance += amount;
  }

  get balance() {
    return this.#balance;
  }

  #validateAmount(amount) {
    if (amount <= 0) throw new Error("Amount must be positive");
  }
}

const acct = new BankAccount(100);
acct.deposit(50);
console.log(acct.balance);   // 150
// acct.#balance;  // SyntaxError

Note Private fields/methods use the # prefix. They are truly private -- not accessible outside the class, even by subclasses. instanceof checks still work.

private fieldprivate method# prefixencapsulationclass privacy

Static Initialization Block

syntax

class C {
  static { /* initialization code */ }
}

example

class Config {
  static defaults;
  static env;

  static {
    Config.env = typeof window !== "undefined" ? "browser" : "node";
    Config.defaults = Config.env === "browser"
      ? { timeout: 10000, retries: 2 }
      : { timeout: 30000, retries: 5 };
  }
}

console.log(Config.env);
console.log(Config.defaults);

Note Static blocks run once when the class is evaluated. Useful for complex static initialization that cannot be done in a single expression.

static blockstatic initializationclass static setup

instanceof and Type Checking

syntax

object instanceof ClassName

example

class Animal {}
class Dog extends Animal {}
class Cat extends Animal {}

const pet = new Dog();
console.log(pet instanceof Dog);    // true
console.log(pet instanceof Animal); // true
console.log(pet instanceof Cat);    // false

Note Checks the prototype chain. Can be unreliable across iframes or realms. For built-in types, prefer Array.isArray() over instanceof Array.

instanceoftype check classcheck class typeprototype chain

Mixins Pattern

syntax

const Mixin = (Base) => class extends Base { ... };

example

const Serializable = (Base) => class extends Base {
  toJSON() {
    return JSON.stringify({ ...this });
  }
};

const Timestamped = (Base) => class extends Base {
  constructor(...args) {
    super(...args);
    this.createdAt = new Date();
  }
};

class User extends Timestamped(Serializable(Object)) {
  constructor(name) {
    super();
    this.name = name;
  }
}

const user = new User("Kai");
console.log(user.toJSON());

Note JavaScript has single inheritance only. Mixins simulate multiple inheritance by composing class factories. Order matters -- later mixins override earlier ones.

mixinmultiple inheritancecompose classesclass composition

Modules

Named Exports

syntax

export const name = value;
export function fn() { ... }
export { a, b, c };

example

// mathUtils.js
export const PI = 3.14159;

export function areaOfCircle(radius) {
  return PI * radius ** 2;
}

export function circumference(radius) {
  return 2 * PI * radius;
}

Note Named exports can be many per module. They must be imported by exact name (or renamed with 'as'). Prefer named exports for better tree-shaking.

named exportexport functionexport variablemodule export

Named Imports

syntax

import { name1, name2 } from "./module.js";
import { name as alias } from "./module.js";

example

import { areaOfCircle, circumference as circ } from "./mathUtils.js";

console.log(areaOfCircle(10));
console.log(circ(10));

Note Imports are live bindings -- they reflect the current value in the exporting module. They are also read-only; you cannot reassign an imported binding.

named importimport functionimport from modulerename import

Default Export and Import

syntax

export default expression;
import anyName from "./module.js";

example

// logger.js
export default class Logger {
  constructor(prefix) {
    this.prefix = prefix;
  }
  log(msg) {
    console.log(`[${this.prefix}] ${msg}`);
  }
}

// app.js
import Logger from "./logger.js";
const log = new Logger("App");
log.log("Started");

output

[App] Started

Note Only one default export per module. Can be imported with any name. You can combine default and named imports: import Logger, { version } from "./logger.js".

default exportdefault importexport defaultimport default

Dynamic Import

syntax

const module = await import("./module.js");

example

async function loadEditor() {
  const { EditorView } = await import("./editor.js");
  return new EditorView(document.getElementById("editor"));
}

// Conditional loading
if (needsCharting) {
  const { renderChart } = await import("./charts.js");
  renderChart(data);
}

Note Returns a promise that resolves to the module namespace. Ideal for code-splitting, lazy loading, and conditional imports. Works at runtime, not just at the top of a file.

dynamic importlazy loadcode splittingconditional importimport()

Re-exporting

syntax

export { name } from "./module.js";
export { default } from "./module.js";
export * from "./module.js";

example

// components/index.js (barrel file)
export { Button } from "./Button.js";
export { Modal } from "./Modal.js";
export { Tooltip } from "./Tooltip.js";

// Consumer
import { Button, Modal } from "./components/index.js";

Note Barrel files simplify imports but can hurt tree-shaking if not handled carefully. Modern bundlers mitigate this, but be aware of the tradeoff.

re-exportbarrel fileexport fromaggregate exports

import.meta

syntax

import.meta.url
import.meta.resolve(specifier)

example

// Get the URL of the current module
console.log(import.meta.url);
// "file:///project/src/utils.js"

// Resolve a relative path
const configPath = new URL("./config.json", import.meta.url);
console.log(configPath.href);

Note import.meta provides module-specific metadata. In Node.js, use import.meta.dirname and import.meta.filename (Node 21+) as replacements for __dirname and __filename.

import.metamodule urlcurrent file pathimport meta url__dirname ESM

Namespace Import

syntax

import * as name from "./module.js";

example

import * as validators from "./validators.js";

const email = "test@example.com";
console.log(validators.isEmail(email));
console.log(validators.isNotEmpty(email));

Note Imports all named exports as a single object. The object is frozen (read-only). Useful when a module has many exports you want to use together.

namespace importimport allimport starwildcard import

Top-Level await

syntax

// In an ES module
const data = await fetchData();

example

// config.js (ES module)
const response = await fetch("/api/config");
export const config = await response.json();

// app.js
import { config } from "./config.js";
console.log(config.appName); // Available immediately

Note Only works in ES modules (not CommonJS). Importing modules that use top-level await will block until that await resolves, so use judiciously.

top level awaitawait in modulemodule async initialization

Error Handling

try / catch / finally

syntax

try { ... }
catch (error) { ... }
finally { ... }

example

function parseConfig(jsonString) {
  try {
    return JSON.parse(jsonString);
  } catch (error) {
    console.error("Invalid config JSON:", error.message);
    return {};
  } finally {
    console.log("Config parsing attempted.");
  }
}

const config = parseConfig('{"debug": true}');
console.log(config);

output

"Config parsing attempted."
{ debug: true }

Note finally always runs, whether the try succeeded or catch was triggered. It even runs if try or catch contains a return statement.

try catcherror handlingfinallycatch errorexception handling

Throwing Errors

syntax

throw new Error(message);
throw new TypeError(message);

example

function withdraw(account, amount) {
  if (amount <= 0) {
    throw new RangeError("Withdrawal amount must be positive");
  }
  if (amount > account.balance) {
    throw new Error("Insufficient funds");
  }
  account.balance -= amount;
  return account.balance;
}

Note Always throw Error objects (not strings) so you get a stack trace. Use specific error types (TypeError, RangeError) when appropriate.

throw errorraise exceptioncustom throwthrow new Error

Custom Error Classes

syntax

class CustomError extends Error {
  constructor(message) {
    super(message);
    this.name = "CustomError";
  }
}

example

class ValidationError extends Error {
  constructor(field, message) {
    super(message);
    this.name = "ValidationError";
    this.field = field;
  }
}

class NotFoundError extends Error {
  constructor(resource, id) {
    super(`${resource} with id ${id} not found`);
    this.name = "NotFoundError";
    this.resource = resource;
    this.id = id;
  }
}

try {
  throw new ValidationError("email", "Invalid email format");
} catch (err) {
  if (err instanceof ValidationError) {
    console.log(`Field: ${err.field}, Message: ${err.message}`);
  }
}

output

"Field: email, Message: Invalid email format"

Note Custom errors let you add context (which field, which resource) and enable precise catch handling with instanceof checks.

custom errorextend Errorerror classtyped errorapplication error

Error Cause (error.cause)

syntax

throw new Error(message, { cause: originalError });

example

async function loadUserData(userId) {
  try {
    const res = await fetch(`/api/users/${userId}`);
    return await res.json();
  } catch (err) {
    throw new Error("Failed to load user data", { cause: err });
  }
}

try {
  await loadUserData(42);
} catch (err) {
  console.error(err.message);        // "Failed to load user data"
  console.error(err.cause?.message);  // original error message
}

Note The cause option (ES2022) lets you wrap errors while preserving the original. Essential for debugging errors that propagate through layers.

error causechain errorswrap errorerror contextoriginal error

Built-in Error Types

syntax

TypeError | RangeError | ReferenceError | SyntaxError | URIError | EvalError | AggregateError

example

// TypeError - wrong type for an operation
try { null.toString(); }
catch (e) { console.log(e.constructor.name); } // "TypeError"

// RangeError - value out of allowed range
try { new Array(-1); }
catch (e) { console.log(e.constructor.name); } // "RangeError"

// ReferenceError - undeclared variable
try { console.log(undeclaredVar); }
catch (e) { console.log(e.constructor.name); } // "ReferenceError"

Note Catching specific types helps you handle known errors differently from unexpected ones. AggregateError wraps multiple errors (used by Promise.any()).

error typesTypeErrorRangeErrorReferenceErrorbuilt-in errors

Optional Catch Binding

syntax

try { ... } catch { ... }

example

function tryParseJSON(str) {
  try {
    return JSON.parse(str);
  } catch {
    return null;
  }
}

console.log(tryParseJSON('{"valid": true}'));  // { valid: true }
console.log(tryParseJSON("not json"));         // null

Note Since ES2019, you can omit the error parameter in catch if you do not need it. Keeps code cleaner for "swallow and fallback" patterns.

optional catchcatch without parameterignore error variable

Centralized Async Error Handling

syntax

process.on('unhandledRejection', handler)
window.addEventListener('unhandledrejection', handler)

example

// Browser global handler
window.addEventListener("unhandledrejection", (event) => {
  console.error("Unhandled rejection:", event.reason);
  event.preventDefault(); // prevents console error
  reportToService(event.reason);
});

// Node.js global handler
process.on("unhandledRejection", (reason, promise) => {
  console.error("Unhandled rejection at:", promise, "reason:", reason);
});

Note Global handlers are your safety net -- they should log and report, not silently swallow errors. Always prefer local try/catch for known async operations.

unhandled rejectionglobal error handlercatch all errorserror reporting

Working with Stack Traces

syntax

error.stack
error.message
error.name

example

function deepFunction() {
  throw new Error("Something broke");
}

try {
  deepFunction();
} catch (err) {
  console.log(err.name);     // "Error"
  console.log(err.message);  // "Something broke"
  console.log(err.stack);    // Full stack trace string
}

Note err.stack is not standardized but is supported everywhere in practice. It includes the error message and the call chain leading to where the error was created.

stack traceerror stackdebug errorerror messageerror properties

DOM Manipulation

Selecting Elements

syntax

document.querySelector(selector)
document.querySelectorAll(selector)
document.getElementById(id)

example

const header = document.querySelector("h1");
const buttons = document.querySelectorAll(".btn");
const main = document.getElementById("main-content");

// querySelectorAll returns a static NodeList
buttons.forEach(btn => {
  console.log(btn.textContent);
});

Note querySelector returns the first match or null. querySelectorAll returns a static NodeList (does not auto-update). Use Array.from() if you need full array methods.

querySelectorselect elementget elementfind DOM elementCSS selector

Creating and Inserting Elements

syntax

document.createElement(tag)
parent.appendChild(child)
parent.append(...nodes)
element.insertAdjacentHTML(position, html)

example

const card = document.createElement("div");
card.className = "card";

const title = document.createElement("h2");
title.textContent = "New Item";

const desc = document.createElement("p");
desc.textContent = "Description goes here.";

card.append(title, desc);
document.getElementById("container").appendChild(card);

Note append() accepts multiple nodes and strings. appendChild() only accepts one node. Prefer textContent over innerHTML to prevent XSS when inserting user-provided text.

create elementadd elementappendChildinsert DOMbuild DOM

Event Listeners

syntax

element.addEventListener(event, handler, options)
element.removeEventListener(event, handler)

example

const button = document.querySelector("#submit-btn");

function handleClick(event) {
  event.preventDefault();
  console.log("Button clicked!", event.target);
}

button.addEventListener("click", handleClick);

// One-time listener
button.addEventListener("click", () => {
  console.log("This fires only once");
}, { once: true });

Note The third argument can be a boolean (useCapture) or an options object { capture, once, passive, signal }. Use { once: true } for one-shot handlers.

event listeneraddEventListenerclick handlerDOM eventhandle event

Event Delegation

syntax

parent.addEventListener(event, (e) => {
  if (e.target.matches(selector)) { ... }
});

example

const todoList = document.querySelector("#todo-list");

todoList.addEventListener("click", (event) => {
  const deleteBtn = event.target.closest(".delete-btn");
  if (deleteBtn) {
    const item = deleteBtn.closest(".todo-item");
    item.remove();
  }
});

Note Attach one listener to a parent instead of many on children. Works for dynamically added elements too. Use closest() to find the nearest matching ancestor.

event delegationdelegate eventsparent listenerdynamic elements eventsclosest

classList Operations

syntax

el.classList.add(cls)
el.classList.remove(cls)
el.classList.toggle(cls)
el.classList.contains(cls)
el.classList.replace(old, new)

example

const panel = document.querySelector(".panel");

panel.classList.add("visible", "animated");
panel.classList.remove("hidden");
panel.classList.toggle("expanded");

if (panel.classList.contains("visible")) {
  console.log("Panel is visible");
}

panel.classList.replace("animated", "static");

Note classList methods are cleaner and safer than manipulating className directly. toggle() returns true if the class was added, false if removed.

classListadd classremove classtoggle classCSS class manipulation

Data Attributes (dataset)

syntax

element.dataset.key
// reads data-key="value" from HTML

example

// HTML: <button data-action="save" data-item-id="42">Save</button>
const btn = document.querySelector("button");

console.log(btn.dataset.action);  // "save"
console.log(btn.dataset.itemId);  // "42" (camelCase conversion)

btn.dataset.status = "pending"; // sets data-status="pending"

Note data-kebab-case becomes camelCase in JavaScript: data-item-id -> dataset.itemId. All dataset values are strings; parse numbers manually.

data attributedatasetcustom attributedata-*read data attribute

innerHTML vs textContent

syntax

element.innerHTML = htmlString;
element.textContent = plainText;

example

const output = document.querySelector("#output");

// textContent: safe, treats everything as text
output.textContent = "<b>Hello</b>"; // shows literal "<b>Hello</b>"

// innerHTML: parses HTML (XSS risk with user input!)
output.innerHTML = "<b>Hello</b>";   // shows bold Hello

// Safe alternative for HTML
const tmpl = document.createElement("template");
tmpl.innerHTML = "<b>Hello</b>";
output.append(tmpl.content.cloneNode(true));

Note NEVER use innerHTML with unsanitized user input -- it is the #1 source of XSS vulnerabilities. Use textContent for plain text and createElement for dynamic HTML.

innerHTMLtextContentXSS preventionset HTMLinsert text

DOM Traversal

syntax

el.parentElement
el.children
el.firstElementChild
el.nextElementSibling
el.closest(selector)

example

const item = document.querySelector(".active-item");

console.log(item.parentElement);        // parent
console.log(item.children);             // HTMLCollection of children
console.log(item.nextElementSibling);   // next sibling element
console.log(item.closest(".container")); // nearest ancestor matching selector

Note Use element-based properties (parentElement, children) over node-based ones (parentNode, childNodes) to skip text/comment nodes.

DOM traversalparent elementchild elementssiblingclosest ancestor

Removing Elements

syntax

element.remove()
parent.removeChild(child)

example

// Modern: element removes itself
const notification = document.querySelector(".notification");
notification.remove();

// Remove all children
const container = document.querySelector("#list");
while (container.firstChild) {
  container.removeChild(container.firstChild);
}

// Faster: clear all children
container.replaceChildren();

Note el.remove() is cleaner than parentNode.removeChild(el). Use replaceChildren() with no arguments to efficiently clear all children.

remove elementdelete DOM elementclear childrenremove from DOM

Modern Features

structuredClone()

syntax

const clone = structuredClone(value);

example

const original = {
  name: "Ava",
  scores: [95, 88, 72],
  metadata: { joined: new Date("2024-01-15") },
};
const clone = structuredClone(original);
clone.scores.push(100);
clone.metadata.joined.setFullYear(2025);

console.log(original.scores);          // [95, 88, 72]
console.log(original.metadata.joined); // 2024-01-15 (unchanged)

Note Built-in deep clone that handles Dates, Maps, Sets, RegExps, ArrayBuffers, circular references. Cannot clone functions, DOM nodes, or Error objects.

structuredClonedeep clonedeep copyclone objectcopy nested

Array/String .at() Method

syntax

arr.at(index)
str.at(index)

example

const stack = ["first", "second", "third", "last"];
console.log(stack.at(0));   // "first"
console.log(stack.at(-1));  // "last"
console.log(stack.at(-2));  // "third"

console.log("hello".at(-1)); // "o"

Note Works on Arrays, Strings, and TypedArrays. The main advantage over bracket notation is support for negative indices.

at methodnegative indexlast elementarray atstring at

findLast() and findLastIndex()

syntax

arr.findLast(callback)
arr.findLastIndex(callback)

example

const transactions = [
  { type: "credit", amount: 500 },
  { type: "debit", amount: 200 },
  { type: "credit", amount: 300 },
  { type: "debit", amount: 150 },
];

const lastCredit = transactions.findLast(t => t.type === "credit");
console.log(lastCredit); // { type: "credit", amount: 300 }

const lastCreditIdx = transactions.findLastIndex(t => t.type === "credit");
console.log(lastCreditIdx); // 2

Note ES2023. Searches from the end of the array. More readable and efficient than reversing first or using a manual reverse loop.

findLastfindLastIndexsearch from endlast matchreverse find

Object.groupBy()

syntax

Object.groupBy(iterable, keyFn)

example

const people = [
  { name: "Ava", department: "Engineering" },
  { name: "Bo", department: "Design" },
  { name: "Cara", department: "Engineering" },
  { name: "Dan", department: "Design" },
  { name: "Eve", department: "Marketing" },
];

const byDept = Object.groupBy(people, p => p.department);
console.log(byDept.Engineering);
// [{ name: "Ava", ... }, { name: "Cara", ... }]
console.log(Object.keys(byDept));
// ["Engineering", "Design", "Marketing"]

Note ES2024. Returns a null-prototype object. For Map output, use Map.groupBy() instead. Replaces the common reduce-based grouping pattern.

groupBygroup arraycategorizeObject.groupBygroup by property

Map.groupBy()

syntax

Map.groupBy(iterable, keyFn)

example

const items = [
  { name: "Apple", price: 1.2 },
  { name: "Steak", price: 15 },
  { name: "Bread", price: 2.5 },
  { name: "Salmon", price: 12 },
];

const byRange = Map.groupBy(items, item =>
  item.price > 10 ? "expensive" : "affordable"
);
console.log(byRange.get("expensive"));
// [{ name: "Steak"... }, { name: "Salmon"... }]

Note ES2024. Similar to Object.groupBy but returns a Map, which supports any key type (objects, symbols, etc.).

Map.groupBygroup into mapcategorize mapgroup by key

Set Methods (ES2025)

syntax

setA.union(setB)
setA.intersection(setB)
setA.difference(setB)
setA.symmetricDifference(setB)
setA.isSubsetOf(setB)
setA.isSupersetOf(setB)
setA.isDisjointFrom(setB)

example

const frontend = new Set(["js", "css", "html", "ts"]);
const backend = new Set(["js", "python", "go", "ts"]);

console.log(frontend.union(backend));
// Set {"js", "css", "html", "ts", "python", "go"}

console.log(frontend.intersection(backend));
// Set {"js", "ts"}

console.log(frontend.difference(backend));
// Set {"css", "html"}

console.log(frontend.isSubsetOf(backend)); // false

Note ES2025. All methods return new Sets (non-mutating). Works with any iterable argument, not just Sets. Replaces manual loop-based set operations.

Set unionSet intersectionSet differenceset methodsset operationsisSubsetOf

Promise.withResolvers()

syntax

const { promise, resolve, reject } = Promise.withResolvers();

example

function createDeferred() {
  const { promise, resolve, reject } = Promise.withResolvers();
  return { promise, resolve, reject };
}

const deferred = createDeferred();

// Resolve from elsewhere
setTimeout(() => deferred.resolve("Done!"), 1000);

const result = await deferred.promise;
console.log(result); // "Done!"

Note ES2024. Eliminates the common pattern of extracting resolve/reject from the Promise constructor. Useful for event-based resolution and deferred patterns.

Promise.withResolversdeferred promiseexternalize resolvecreate deferred

Iterator Helpers (ES2025)

syntax

iterator.map(fn)
iterator.filter(fn)
iterator.take(n)
iterator.drop(n)
iterator.flatMap(fn)
iterator.toArray()

example

function* naturals() {
  let n = 1;
  while (true) yield n++;
}

// Get first 5 even squares
const result = naturals()
  .map(n => n ** 2)
  .filter(n => n % 2 === 0)
  .take(5)
  .toArray();

console.log(result); // [4, 16, 36, 64, 100]

Note ES2025. Chainable lazy operations on any iterator. Unlike array methods, these process elements one at a time, making them memory-efficient for large or infinite sequences.

iterator helperslazy iterationiterator mapiterator filteriterator takeiterator drop

Explicit Resource Management (using)

syntax

using resource = acquireResource();
await using resource = acquireAsyncResource();

example

class TempFile {
  constructor(path) {
    this.path = path;
    console.log(`Created: ${path}`);
  }

  [Symbol.dispose]() {
    console.log(`Cleaned up: ${this.path}`);
  }
}

{
  using file = new TempFile("/tmp/data.txt");
  // Use file here...
} // Automatically calls file[Symbol.dispose]() when block exits

output

"Created: /tmp/data.txt"
"Cleaned up: /tmp/data.txt"

Note TC39 Stage 3 (expected in ES2025/2026). Similar to Python's 'with' or C#'s 'using'. Guarantees cleanup even if an error is thrown. Use Symbol.asyncDispose for async cleanup.

using keywordSymbol.disposeresource managementauto cleanupdisposable

RegExp v Flag (Unicode Sets)

syntax

/pattern/v

example

// Match any emoji using Unicode property
const emojiPattern = /\p{Emoji_Presentation}/v;
console.log(emojiPattern.test("hello")); // false
console.log(emojiPattern.test("hi [U+D83D][U+DC4B]")); // true

// Set operations in character classes
const greekLetters = /[\p{Script=Greek}&&\p{Letter}]/v;
console.log(greekLetters.test("α")); // true

Note ES2024. The v flag replaces the u flag with enhanced Unicode support and set operations (&&, --) inside character classes.

regex v flagunicode setsregexp unicodecharacter class operations

Common Patterns

Debounce

syntax

function debounce(fn, delay) { ... }

example

function debounce(fn, delay) {
  let timerId;
  return function (...args) {
    clearTimeout(timerId);
    timerId = setTimeout(() => fn.apply(this, args), delay);
  };
}

// Usage: only fire after user stops typing for 300ms
const searchInput = document.querySelector("#search");
searchInput.addEventListener("input",
  debounce((e) => {
    console.log("Searching:", e.target.value);
  }, 300)
);

Note Debouncing delays execution until activity stops for a given period. Ideal for search inputs, window resize handlers, and form validation.

debouncedelay functionsearch input debouncerate limit input

Throttle

syntax

function throttle(fn, interval) { ... }

example

function throttle(fn, interval) {
  let lastTime = 0;
  return function (...args) {
    const now = Date.now();
    if (now - lastTime >= interval) {
      lastTime = now;
      fn.apply(this, args);
    }
  };
}

// Usage: fire at most once per 200ms during scroll
window.addEventListener("scroll",
  throttle(() => {
    console.log("Scroll position:", window.scrollY);
  }, 200)
);

Note Throttling ensures a function runs at most once per interval, even if triggered constantly. Use for scroll, mousemove, and resize events.

throttlerate limitlimit callsscroll handler throttle

Memoization

syntax

function memoize(fn) { ... }

example

function memoize(fn) {
  const cache = new Map();
  return function (...args) {
    const key = JSON.stringify(args);
    if (cache.has(key)) return cache.get(key);
    const result = fn.apply(this, args);
    cache.set(key, result);
    return result;
  };
}

const expensiveCalc = memoize((n) => {
  console.log("Computing...");
  return n ** 2 + Math.sqrt(n);
});

console.log(expensiveCalc(100)); // "Computing..." then 10010
console.log(expensiveCalc(100)); // 10010 (cached, no log)

Note Caches results for repeated calls with the same arguments. Use JSON.stringify for simple keys. For complex args, consider a WeakMap-based approach.

memoizecache functionmemoizationcache resultsavoid recalculation

Currying

syntax

const curried = (a) => (b) => (c) => result;

example

const createFormatter = (currency) => (locale) => (amount) =>
  new Intl.NumberFormat(locale, {
    style: "currency", currency
  }).format(amount);

const formatUSD = createFormatter("USD")("en-US");
const formatEUR = createFormatter("EUR")("de-DE");

console.log(formatUSD(1234.5));  // "$1,234.50"
console.log(formatEUR(1234.5));  // "1.234,50 €"

Note Currying transforms a function of multiple arguments into a chain of single-argument functions. Enables partial application and reusable configurations.

currycurryingpartial applicationfunction composition

Singleton Pattern

syntax

class Singleton { static #instance; static getInstance() {} }

example

class Database {
  static #instance;
  #connection;

  constructor() {
    if (Database.#instance) {
      throw new Error("Use Database.getInstance()");
    }
    this.#connection = "connected";
  }

  static getInstance() {
    if (!Database.#instance) {
      Database.#instance = new Database();
    }
    return Database.#instance;
  }

  query(sql) {
    console.log(`[${this.#connection}] Executing: ${sql}`);
  }
}

const db1 = Database.getInstance();
const db2 = Database.getInstance();
console.log(db1 === db2); // true

Note Ensures only one instance of a class exists. In modern JavaScript, ES modules are natural singletons -- an exported object is shared across all importers.

singletonsingle instancesingleton patternshared instance

Observer / Event Emitter Pattern

syntax

class EventEmitter { on(event, handler) {} emit(event, data) {} }

example

class EventBus {
  #handlers = new Map();

  on(event, handler) {
    if (!this.#handlers.has(event)) {
      this.#handlers.set(event, new Set());
    }
    this.#handlers.get(event).add(handler);
    return () => this.off(event, handler); // unsubscribe fn
  }

  off(event, handler) {
    this.#handlers.get(event)?.delete(handler);
  }

  emit(event, data) {
    this.#handlers.get(event)?.forEach(fn => fn(data));
  }
}

const bus = new EventBus();
const unsub = bus.on("user:login", (user) => console.log(`Welcome, ${user}`));
bus.emit("user:login", "Ava"); // "Welcome, Ava"
unsub(); // unsubscribes

Note The observer pattern decouples event producers from consumers. Returning an unsubscribe function from on() prevents memory leaks.

observer patternevent emitterpub subevent bussubscribe unsubscribe

Pipe / Compose

syntax

const pipe = (...fns) => (x) => fns.reduce((v, fn) => fn(v), x);

example

const pipe = (...fns) => (x) => fns.reduce((v, fn) => fn(v), x);

const sanitize = pipe(
  (str) => str.trim(),
  (str) => str.toLowerCase(),
  (str) => str.replace(/[^a-z0-9\s-]/g, ""),
  (str) => str.replace(/\s+/g, "-")
);

console.log(sanitize("  Hello World! @2026  "));
// "hello-world-2026"

Note pipe() applies functions left-to-right; compose() applies right-to-left. Pipe is more intuitive for data transformation chains.

pipecomposefunction pipelinechain functionsdata transformation

Retry with Exponential Backoff

syntax

async function retry(fn, attempts, delay) { ... }

example

async function retry(fn, maxAttempts = 3, baseDelay = 1000) {
  for (let attempt = 1; attempt <= maxAttempts; attempt++) {
    try {
      return await fn();
    } catch (err) {
      if (attempt === maxAttempts) throw err;
      const delay = baseDelay * 2 ** (attempt - 1);
      console.log(`Attempt ${attempt} failed, retrying in ${delay}ms...`);
      await new Promise(r => setTimeout(r, delay));
    }
  }
}

// Usage
const data = await retry(() => fetch("/api/flaky-endpoint").then(r => r.json()));

Note Exponential backoff (1s, 2s, 4s...) prevents overwhelming failing services. Add jitter (random offset) in production to avoid thundering herd.

retry patternexponential backoffretry fetchresilient request

Common Mistakes

== vs === Confusion

syntax

// Wrong: == with type coercion
// Right: === for strict comparison

example

// These all evaluate to true (unexpected!)
console.log("" == false);    // true
console.log(0 == "");        // true
console.log(null == undefined); // true
console.log("0" == false);   // true

// Use strict equality
console.log("" === false);   // false
console.log(0 === "");       // false
console.log(null === undefined); // false

Note Rule of thumb: always use === and !==. The only acceptable == use is value == null to check both null and undefined at once.

== vs ===loose equalitystrict equalityequality bugcomparison mistake

Losing 'this' Context

syntax

// Problem: extracting a method loses its 'this'
// Fix: use bind(), arrow function, or keep method call

example

class Timer {
  count = 0;

  // BUG: 'this' is undefined in the callback
  startBroken() {
    setInterval(function () {
      this.count++; // TypeError!
    }, 1000);
  }

  // FIX 1: arrow function inherits 'this'
  startFixed() {
    setInterval(() => {
      this.count++;
      console.log(this.count);
    }, 1000);
  }
}

// FIX 2: bind the method
const timer = new Timer();
const increment = timer.startFixed.bind(timer);

Note Arrow functions, .bind(), or storing this in a variable (const self = this) all solve this. Arrow functions are the cleanest modern solution.

this bindinglost thisthis undefinedcontext bindingarrow function this

Closure in Loops (var trap)

syntax

// Bug: var is function-scoped
// Fix: use let (block-scoped)

example

// BUG: all callbacks log 3
for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Logs: 3, 3, 3

// FIX: let is block-scoped, each iteration gets its own i
for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Logs: 0, 1, 2

Note With var, all iterations share the same variable. By the time callbacks run, the loop is done and i has its final value. This is the #1 reason to use let.

closure in loopvar loop bugsetTimeout looploop variable capture

Floating Point Precision

syntax

// 0.1 + 0.2 !== 0.3

example

console.log(0.1 + 0.2);         // 0.30000000000000004
console.log(0.1 + 0.2 === 0.3); // false

// FIX 1: Compare with tolerance
function nearlyEqual(a, b, epsilon = Number.EPSILON) {
  return Math.abs(a - b) < epsilon;
}
console.log(nearlyEqual(0.1 + 0.2, 0.3)); // true

// FIX 2: Work in integers (cents instead of dollars)
const priceInCents = 199; // $1.99
const total = priceInCents * 3; // 597 cents = $5.97

Note All numbers in JavaScript are 64-bit floating point (IEEE 754). For financial calculations, use integer arithmetic (cents) or a decimal library.

floating point0.1 + 0.2precision errordecimal roundingmoney calculation

Unexpected Array Mutation

syntax

// Mutating: sort, reverse, splice, push, pop, shift, unshift
// Non-mutating: toSorted, toReversed, toSpliced, with, map, filter, slice

example

const original = [3, 1, 2];
const sorted = original.sort(); // BUG: mutates original!
console.log(original); // [1, 2, 3] (changed!)
console.log(sorted === original); // true (same reference!)

// FIX 1: Copy first
const safeSort = [...original].sort();

// FIX 2: Use non-mutating method (ES2023)
const safeSorted = original.toSorted();

Note sort(), reverse(), splice() all mutate. This is a major bug source especially in React/Vue state. Use toSorted(), toReversed(), toSpliced() for safe alternatives.

array mutationsort mutatesunexpected mutationimmutable array operations

async/await with forEach (Does Not Work)

syntax

// BUG: forEach ignores async callbacks
// FIX: use for...of or Promise.all with map

example

const userIds = [1, 2, 3];

// BUG: forEach does NOT await each call
userIds.forEach(async (id) => {
  const user = await fetch(`/api/users/${id}`);
  console.log(await user.json()); // fires unpredictably
});

// FIX 1: Sequential processing
for (const id of userIds) {
  const user = await fetch(`/api/users/${id}`);
  console.log(await user.json());
}

// FIX 2: Parallel processing
const users = await Promise.all(
  userIds.map(id => fetch(`/api/users/${id}`).then(r => r.json()))
);

Note forEach returns undefined and does not await the callback. The loop completes immediately while async callbacks run in the background with no error handling.

async forEachforEach async bugawait in loopasync array iteration

Object/Array Reference Sharing

syntax

// Assigning objects copies the reference, not the value

example

const defaults = { theme: "light", notifications: { email: true, sms: false } };

// BUG: both point to the same object
const userSettings = defaults;
userSettings.theme = "dark";
console.log(defaults.theme); // "dark" (changed!)

// FIX: shallow copy
const settings = { ...defaults };

// FIX: deep copy (for nested objects)
const deepSettings = structuredClone(defaults);

Note Assignment (=) with objects copies the reference, not the data. Spread (...) is only a shallow copy. Use structuredClone() for nested structures.

reference vs copyobject referenceshallow copy bugshared referencecopy object

typeof null and Other Gotchas

syntax

typeof null === "object"  // true (historic bug)
typeof NaN === "number"   // true (NaN is a number type)
typeof [] === "object"    // true (arrays are objects)

example

// All of these are surprising but correct
console.log(typeof null);        // "object"
console.log(typeof NaN);         // "number"
console.log(typeof []);          // "object"
console.log(typeof function(){}); // "function"

// Better checks
console.log(value === null);           // null check
console.log(Number.isNaN(value));      // NaN check
console.log(Array.isArray(value));     // array check

Note typeof is unreliable for null, arrays, and NaN. Use specialized checks: === null, Array.isArray(), Number.isNaN(), or instanceof for specific types.

typeof nulltypeof gotchacheck nullcheck arraytypeof quirks