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Learn all JavaScript keywords and reserved words: var, let, const, if, else, for, while, function, return, class, and more. Full list with examples and interview Q&A.
JavaScript keywords are special words that are part of the language syntax itself. They have pre-defined meanings and cannot be used as variable names, function names, or identifiers. Learning them helps you understand JavaScript's grammar and avoid confusing syntax errors.
Real World Analogy
Imagine a game with fixed commands: START, STOP, JUMP, RUN. These are the game's keywords — you can't name your character "START" because the game would get confused. JavaScript works the same way: if, let, return are the engine's commands, and you can't use them as your own variable names.
How Keywords Work Internally
Example
JavaScript Source Code
│
▼
┌──────────────────────────────────────┐
│ TOKENIZER / LEXER │
│ │
│ Scans text character by character │
│ │
│ "let" ──► KEYWORD token │
│ "x" ──► IDENTIFIER token │
│ "=" ──► OPERATOR token │
│ "5" ──► LITERAL token │
│ ";" ──► PUNCTUATION token │
└──────────────────────────────────────┘
│
▼
┌──────────────────────────────────────┐
│ PARSER builds the AST │
│ (keyword tokens have special roles │
│ in the grammar rules) │
└──────────────────────────────────────┘
Keywords are recognized in the very first phase (tokenization) and given special grammar rules during parsing.
Complete List of JavaScript Keywords
Declaration Keywords
Used to declare variables, functions, and classes:
text example
var let const
function class js example
var score = 100;
let name = "Aisha";
const PI = 3.14159;
function greet(person) {
return "Hello, " + person;
}
class Animal {
constructor(type) {
this.type = type;
}
}
console.log(greet(name));
console.log(new Animal("Dog").type);Output
Hello, Aisha Dog
Control Flow Keywords
Used to make decisions and control the direction of code execution:
Example
if else switch
case default break
continue return
js example
const score = 75;
if (score >= 90) {
console.log("Grade: A");
} else if (score >= 75) {
console.log("Grade: B");
} else {
console.log("Grade: C");
}Output
Grade: B
Example
// switch-case example
const day = 3;
switch (day) {
case 1:
console.log("Monday");
break;
case 2:
console.log("Tuesday");
break;
case 3:
console.log("Wednesday");
break;
default:
console.log("Other day");
}
Output
Wednesday
Loop Keywords
Used to repeat code:
text example
for while do
break continue Example
// for loop
for (let i = 1; i <= 3; i++) {
console.log("Step", i);
}
Output
Step 1 Step 2 Step 3
Example
// while with continue and break
let count = 0;
while (count < 10) {
count++;
if (count === 5) continue; // skip 5
if (count === 8) break; // stop at 8
console.log(count);
}
Output
1 2 3 4 6 7
Object & Class Keywords
text example
new this super
extends static instanceof
in delete typeof
void js example
class Vehicle {
constructor(brand) {
this.brand = brand; // 'this' refers to the new object
}
describe() {
return `I am a ${this.brand}`;
}
}
class Car extends Vehicle {
constructor(brand, doors) {
super(brand); // 'super' calls the parent class constructor
this.doors = doors;
}
}
const myCar = new Car("Toyota", 4);
console.log(myCar.describe());
console.log(myCar instanceof Car);
console.log(myCar instanceof Vehicle);
console.log("brand" in myCar);Output
I am a Toyota true true true
Error Handling Keywords
text example
try catch finally
throw js example
function divide(a, b) {
if (b === 0) {
throw new Error("Cannot divide by zero!");
}
return a / b;
}
try {
console.log(divide(10, 2));
console.log(divide(10, 0)); // throws!
} catch (error) {
console.log("Caught:", error.message);
} finally {
console.log("This always runs");
}Output
5 Caught: Cannot divide by zero! This always runs
Module Keywords (ES6+)
Used for modular code organization:
text example
import export from
as default Example
// file: math.js
export const add = (a, b) => a + b;
export default function multiply(a, b) { return a * b; }
// file: app.js
import multiply, { add } from './math.js';
import * as MathUtils from './math.js';
Async Programming Keywords (ES2017+)
text example
async await js example
async function fetchData() {
try {
const response = await fetch("https://api.example.com/users");
const data = await response.json();
console.log(data);
} catch (error) {
console.log("Fetch failed:", error.message);
}
}Other Important Keywords
text example
typeof instanceof void
delete in yield
debugger with js example
console.log(typeof "hello"); // "string"
console.log(typeof undefined); // "undefined"
console.log(void 0); // undefined (used to get undefined safely)
const obj = { a: 1, b: 2, c: 3 };
delete obj.b;
console.log(obj);Output
string
undefined
undefined
{ a: 1, c: 3 }All JavaScript Keywords at a Glance
Example
┌─────────────────────────────────────────────────────────────┐
│ JAVASCRIPT KEYWORDS (ES2024) │
├──────────────┬──────────────┬──────────────┬────────────────┤
│ Declaration │ Control │ Loops │ Error Handling │
│ ──────────── │ ──────────── │ ──────────── │ ───────────── │
│ var │ if │ for │ try │
│ let │ else │ while │ catch │
│ const │ switch │ do │ finally │
│ function │ case │ break │ throw │
│ class │ default │ continue │ │
├──────────────┼──────────────┼──────────────┼────────────────┤
│ OOP │ Modules │ Async │ Operators │
│ ──────────── │ ──────────── │ ──────────── │ ───────────── │
│ new │ import │ async │ typeof │
│ this │ export │ await │ instanceof │
│ super │ from │ yield │ in │
│ extends │ as │ │ delete │
│ static │ default │ │ void │
└──────────────┴──────────────┴──────────────┴────────────────┘
Future Reserved Words (cannot be used even if not active yet):
enum implements interface package private protected public
Keywords vs Identifiers vs Literals
Example
let score = 100 ;
↑ ↑ ↑ ↑ ↑
│ │ │ │ └── Punctuation
│ │ │ └────── Numeric Literal (you define)
│ │ └────────── Operator
│ └─────────────── Identifier (you name this)
└───────────────────── KEYWORD (JavaScript owns this)
| Token Type | Who Owns It | Examples |
|---|---|---|
| Keyword | JavaScript | let, if, for, return |
| Identifier | You (the developer) | score, userName, myFunction |
| Literal | You (the value) | "hello", 42, true, null |
| Operator | JavaScript | +, -, ===, typeof |
Rules for Naming Variables (Avoiding Keywords)
Example
// ❌ These will throw SyntaxError:
// let let = 5;
// const function = () => {};
// var class = "myClass";
// let return = 10;
// ✅ These are fine:
let letCount = 5; // contains "let" but isn't exactly "let"
const functionName = "greet"; // contains "function" as part of identifier
let className = "myClass"; // fine — "className" is not a keyword
Common Mistakes
❌ Mistake 1: Using a keyword as a variable name
js example
// ❌ SyntaxError
let new = "value"; // 'new' is a keyword
let class = "ES6"; // 'class' is a keywordjs example
// ✅ Add a descriptive prefix or suffix
let newValue = "value";
let className = "ES6";❌ Mistake 2: Confusing in the operator with iteration
Example
const arr = [10, 20, 30];
// ❌ 'for...in' on arrays gives INDEX not value
for (let key in arr) {
console.log(key); // "0", "1", "2" — these are indices, not values!
}
js example
// ✅ Use 'for...of' for array values
for (let value of arr) {
console.log(value); // 10, 20, 30
}❌ Mistake 3: Forgetting return in a function
js example
// ❌ No return — function returns undefined
function add(a, b) {
a + b; // calculated but never returned!
}
console.log(add(3, 4)); // undefinedjs example
// ✅ Use the return keyword
function add(a, b) {
return a + b;
}
console.log(add(3, 4)); // 7Key Takeaways
- JavaScript keywords are reserved words — they have fixed meanings in the language grammar
- You cannot use keywords as variable names — doing so causes a
SyntaxError - Keywords fall into categories: declaration, control flow, loops, OOP, error handling, modules, async
- Future reserved words (like
enum) also cannot be used as identifiers in strict mode - Keywords like
typeof,instanceof,in,delete,voidare also operators - Understanding keywords helps you read JavaScript syntax more fluently
Interview Questions & Answers
Q1. What is a keyword in JavaScript?
Answer: A keyword is a reserved word that has a special, pre-defined meaning in the JavaScript language syntax. Keywords form the grammar of the language (like let for variable declaration, if for conditionals, return to exit a function). You cannot use keywords as variable names, function names, or property names (in some cases).
Q2. What happens if you use a keyword as a variable name?
Answer: You get a SyntaxError at parse time. For example, let return = 5; throws SyntaxError: Unexpected token 'return'. The JavaScript parser recognizes return as a keyword and doesn't allow it as an identifier.
Q3. What is the difference between a keyword, an identifier, and a literal?
Answer:
- Keyword: Reserved by JavaScript, cannot be redefined (e.g.,
let,if,class) - Identifier: A name you choose for variables, functions, parameters (e.g.,
userName,calculateTotal) - Literal: An actual value written directly in code (e.g.,
42,"hello",true,null)
Q4. What are "future reserved words" in JavaScript?
Answer: Future reserved words are identifiers that are currently reserved for potential future use in the language. Examples: enum, implements, interface, package, private, protected, public. In strict mode, using these as identifiers throws an error even though they have no current implementation.
Q5. Is undefined a keyword in JavaScript?
Answer: No! undefined is not a keyword — it's a global property of the global object. This means in very old JavaScript (pre-ES5) you could technically reassign it (undefined = 5), which is why void 0 was sometimes used to reliably get undefined. In modern code (strict mode), undefined cannot be reassigned in local scope.
Q6. What is the in keyword used for?
Answer: The in operator checks if a property exists in an object (returns true/false). It's also used in for...in loops to iterate over object property keys. Example: "name" in user returns true if the object has a name property.
Q7. What is the void keyword in JavaScript?
Answer: void is an operator that evaluates an expression and returns undefined. It's most commonly used as void 0 to get a reliable undefined value. It also appears in HTML: <a href="javascript:void(0)"> — a pattern that prevents page navigation when clicking a link (though modern code uses event.preventDefault() instead).
Q8. Can this be used as a variable name?
Answer: No. this is a keyword in JavaScript and cannot be used as a variable name. It is a special contextual reference that refers to the current execution context (the object that owns the method, or the global object in non-strict mode, or undefined in strict mode functions).
Exam Focus
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