Skip to main content

Keywords

Definition​

A keyword is a special word that is part of JavaScript's syntax. They are not functions, methods, or objects. They define how the language is structured and executed.

CategoryExamplesDescription
Control flowif, else, for, while, do, break, continue, return, switch, case, defaultManage loops and decision-making
Structure and definitionsfunction, class, const, let, var, import, exportDefine functions, classes, and variables
Logical and comparisontypeof, instanceof, in, void, deleteType checking and property operations
Exception handlingtry, catch, finally, throwHandle errors
Object-orientednew, this, super, extends, staticObject creation and inheritance
Asynchronousasync, await, yieldAsynchronous programming
Special purposenull, undefined, true, false, debugger, withLanguage-level constants or syntax keywords

Keyword definitions​

if​

Starts a conditional block that executes code only when a condition is true. The bread and butter of decision-making.

if (x > 0) {
  console.log("positive")
}

else​

Executes code when the preceding if condition is false. The "otherwise" clause.

if (x > 0) {
  console.log("positive")
} else {
  console.log("not positive")
}

for​

Creates a loop that executes a block of code a specified number of times.

for (let i = 0; i < 5; i++) {
  console.log(i)
}

while​

Creates a loop that executes a block of code as long as a condition is true.

let i = 0
while (i < 5) {
  console.log(i)
  i++
}

do​

Creates a loop that executes at least once, then continues while a condition is true.

let i = 0
do {
  console.log(i)
  i++
} while (i < 5)

break​

Exits a loop or switch statement immediately.

for (let i = 0; i < 10; i++) {
  if (i === 5) break
  console.log(i)
}

continue​

Skips the rest of the current loop iteration and continues with the next iteration.

for (let i = 0; i < 10; i++) {
  if (i % 2 === 0) continue
  console.log(i)  // Only odd numbers
}

return​

Exits a function and optionally returns a value.

function add(a, b) {
  return a + b
}

switch​

Evaluates an expression and executes code based on matching cases.

switch (value) {
  case 1:
    console.log("one")
    break
  case 2:
    console.log("two")
    break
  default:
    console.log("other")
}

case​

Defines a match condition in a switch statement.

default​

Defines the default case in a switch statement when no other cases match.

function​

Declares a function.

function greet(name) {
  return `Hello, ${name}!`
}

class​

Declares a class for object-oriented programming.

class Person {
  constructor(name) {
    this.name = name
  }
}

const​

Declares a constant variable that cannot be reassigned.

const PI = 3.14159

let​

Declares a block-scoped variable that can be reassigned.

let x = 5
x = 10  // OK

var​

Declares a function-scoped variable (legacy, prefer let or const).

var x = 5

import​

Imports bindings from another module.

import { add } from './math.js'
import React from 'react'

export​

Exports bindings from a module.

export function add(a, b) {
  return a + b
}

export const PI = 3.14159

try​

Starts a block of code to test for errors.

try {
  riskyOperation()
} catch (error) {
  console.error(error)
}

catch​

Handles errors thrown in a try block.

try {
  throw new Error("Something went wrong")
} catch (error) {
  console.error(error.message)
}

finally​

Executes code after try and catch, regardless of the result.

try {
  // code
} catch (error) {
  // handle error
} finally {
  // always executes
}

throw​

Throws a user-defined exception.

throw new Error("Something went wrong")

typeof​

Returns a string indicating the type of a value.

typeof 42           // "number"
typeof "hello"      // "string"
typeof true         // "boolean"
typeof undefined    // "undefined"
typeof null         // "object" (quirk)

instanceof​

Tests whether an object is an instance of a constructor.

[] instanceof Array        // true
[] instanceof Object       // true
new Date() instanceof Date // true

in​

Tests whether a property exists in an object.

'name' in { name: 'Alice' }  // true
'age' in { name: 'Alice' }   // false

new​

Creates an instance of a constructor function or class.

new Date()
new Array(5)
new Person('Alice')

this​

Refers to the current object context.

const person = {
  name: 'Alice',
  greet() {
    return `Hello, I'm ${this.name}`
  }
}

super​

Calls the parent class constructor or method.

class Animal {
  speak() {
    return "Some sound"
  }
}

class Dog extends Animal {
  speak() {
    return super.speak() + " - Woof!"
  }
}

extends​

Creates a class that inherits from another class.

class Dog extends Animal {
  // ...
}

static​

Defines a static method or property on a class.

class Math {
  static add(a, b) {
    return a + b
  }
}

async​

Declares an asynchronous function.

async function fetchData() {
  const response = await fetch('/api/data')
  return response.json()
}

await​

Pauses execution until a Promise is resolved.

async function example() {
  const data = await fetch('/api/data')
  return data.json()
}

yield​

Pauses and resumes a generator function.

function* numbers() {
  yield 1
  yield 2
  yield 3
}

void​

Evaluates an expression and returns undefined.

void 0  // undefined

delete​

Removes a property from an object.

const obj = { a: 1, b: 2 }
delete obj.a
// obj is now { b: 2 }

debugger​

Invokes any available debugging functionality.

function problematicFunction() {
  debugger  // Execution pauses here if debugger is active
  // ...
}

with (deprecated)​

Extends the scope chain for a statement. Deprecated: Do not use. It's not allowed in strict mode and makes code harder to understand.

enum, implements, interface, package, private, protected, public​

These are reserved words in JavaScript but not currently used. They are reserved for potential future use or for compatibility with other languages (like TypeScript).