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Master your JavaScript interview with our comprehensive guide featuring expert questions and answers for front-end and full-stack developers. Covering ES6, closures, promises, async/await, and more. Free PDF download available.
Download the full PDF with all JavaScript interview questions and expert answers
Download PDF NowWhen interviewing for a JavaScript developer position, you'll face questions about core language concepts, asynchronous programming, DOM manipulation, closures, prototypes, and modern ES6+ features. The hiring manager will want to assess both your theoretical knowledge and practical coding ability.
Prepare for the interview by reviewing the job requirements and practicing coding problems. Take the job requirements from the posting and match your skills. Be ready to discuss projects you've built and problems you've solved with JavaScript.
Review this list of JavaScript interview questions and take time to prepare responses based on your experience. When responding, give specific code examples and explain your reasoning clearly.
var, let, and const?var: Function-scoped, can be redeclared, hoisted and initialized with undefined. Avoid in modern JS.let: Block-scoped, cannot be redeclared in same scope, hoisted but not initialized (Temporal Dead Zone).const: Block-scoped, cannot be reassigned, must be initialized at declaration. For objects, properties can still be modified.var x = 1; // function-scoped
let y = 2; // block-scoped
const z = 3; // block-scoped, can't reassign
== and ===?== (Abstract Equality): Compares values after type coercion. 5 == '5' returns true.=== (Strict Equality): Compares both value and type without coercion. 5 === '5' returns false.=== unless you specifically need type coercion.
var variables: Hoisted and initialized with undefinedlet and const: Hoisted but not initialized (Temporal Dead Zone)console.log(myVar); // undefined (not an error)
var myVar = 5;
console.log(myLet); // ReferenceError
let myLet = 5;
null and undefined?undefined: A variable has been declared but not assigned a value. Also the default return value of functions with no return statement.null: An intentionally assigned empty value, representing "nothing" or "empty".typeof undefined // "undefined"
typeof null // "object" (historical bug in JS)
function outer(x) {
return function inner(y) {
return x + y;
};
}
const add5 = outer(5);
console.log(add5(3)); // 8 (closure remembers x=5)var inside a function. Accessible only within that function.let and const inside blocks {}. Accessible only within that block.callbacks, Promises, and async/await?.then() and .catch(). Avoid callback hell.async functions return a Promise; await pauses execution until Promise resolves.async function fetchData() {
try {
const response = await fetch(url);
const data = await response.json();
return data;
} catch(error) {
console.error(error);
}
}
setTimeout, event listeners, and Promises to run without blocking the main thread.
() => {}this binding (lexical this - inherits from parent scope)new keyword)arguments objectthisconst add = (a, b) => a + b;
const square = x => x * x;
const [a, b] = [1, 2]; // a=1, b=2const {name, age} = {name: 'John', age: 30};
// name='John', age=30...): Expands an iterable into individual elements.const arr1 = [1, 2];
const arr2 = [...arr1, 3, 4]; // [1,2,3,4]...): Collects multiple elements into an array.function sum(...numbers) {
return numbers.reduce((a,b) => a+b);
}document.getElementById() - Select by IDdocument.querySelector() - Select with CSS selectorelement.innerHTML - Get/set HTML contentelement.addEventListener() - Attach event handlerselement.classList - Manipulate CSS classesconst parent = { greet() { return "Hello"; } };
const child = Object.create(parent);
console.log(child.greet()); // "Hello" (inherited)map(), filter(), and reduce()?map(): Creates a new array by applying a function to each element.[1,2,3].map(x => x*2) // [2,4,6]filter(): Creates a new array with elements that pass a test.[1,2,3,4].filter(x => x%2===0) // [2,4]reduce(): Reduces array to a single value by applying a function.[1,2,3].reduce((sum, x) => sum + x, 0) // 6console.log(), console.table(), console.time() for loggingdebugger; statement to pause executionGet the complete JavaScript Interview Questions and Answers guide in PDF format
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