Here are some commonly asked JavaScript interview questions along with their answers:
1. What is the difference between var, let, and const?
var: Function-scoped, can be redeclared, hoisted but not block-scoped.
let: Block-scoped, cannot be redeclared within the same block, hoisted but not initialized.
const: Block-scoped, must be initialized at the time of declaration, value cannot be reassigned.
Example:
CODE
function example() {
if (true) {
var x = 10; // accessible outside this block
let y = 20; // block-scoped
const z = 30; // block-scoped and immutable
}
console.log(x); // 10
// console.log(y); // Error
// console.log(z); // Error
}
2. What is a closure, and how is it used?
- A closure is when a function "remembers" the variables from its lexical scope even when the function is executed outside that scope.
Example:
CODE
function outer() {
let count = 0;
return function inner() {
count++;
return count;
};
}
const counter = outer();
console.log(counter()); // 1
console.log(counter()); // 2
console.log(counter()); // 3
3. Explain the difference between == and ===.
==: Compares values after type coercion.
===: Compares values and types strictly without type coercion.
Example:
CODE
console.log(5 == '5'); // true (type coercion)
console.log(5 === '5'); // false (strict comparison)
4. How does the JavaScript Event Loop work?
- The event loop allows JavaScript to perform non-blocking operations by using a single-threaded model.
Call Stack: Executes code.
Task Queue: Stores asynchronous callbacks likesetTimeout.
Microtask Queue: Stores promises (.then,catch).
Event Loop: Monitors the stack and pushes tasks from the queue when the stack is empty.
Example:
CODE
console.log('Start');
setTimeout(() => console.log('Timeout'), 0);
Promise.resolve().then(() => console.log('Promise'));
console.log('End');
// Output: Start, End, Promise, Timeout
5. What is the purpose of bind(), call(), and apply()?
These methods are used to change the this context of a function:
bind(): Returns a new function with the specifiedthis.
call(): Invokes the function immediately with a specifiedthisand arguments.
apply(): Similar tocall()but takes arguments as an array.
Example:
CODE
const obj = { name: 'Alice' };
function greet(greeting) {
return `${greeting}, ${this.name}`;
}
console.log(greet.call(obj, 'Hello')); // Hello, Alice
console.log(greet.apply(obj, ['Hi'])); // Hi, Alice
const boundGreet = greet.bind(obj);
console.log(boundGreet('Hey')); // Hey, Alice
6. Write a function to check if a string is a palindrome.
A palindrome reads the same backward as forward.
CODE
function isPalindrome(str) {
const reversed = str.split('').reverse().join('');
return str === reversed;
}
console.log(isPalindrome('madam')); // true
console.log(isPalindrome('hello')); // false
7. What is the difference between Object.create() and using the new keyword?
Object.create(): Creates a new object with a specified prototype.
newkeyword: Creates an instance of a constructor function and sets the prototype.
Example:
CODE
const parent = { greet: () => 'Hello' };
const child = Object.create(parent);
console.log(child.greet()); // Hello
function Person(name) {
this.name = name;
}
const person = new Person('John');
console.log(person.name); // John
8. Implement a custom Array.prototype.map function.
CODE
Array.prototype.myMap = function(callback) {
const result = [];
for (let i = 0; i < this.length; i++) {
result.push(callback(this[i], i, this));
}
return result;
};
const arr = [1, 2, 3];
const doubled = arr.myMap(num => num * 2);
console.log(doubled); // [2, 4, 6]
9. How do debouncing and throttling work?
Debouncing: Ensures a function is executed only after a specified delay after the last call.
Throttling: Ensures a function is executed at most once in a specified interval.
Debouncing Example:
CODE
function debounce(fn, delay) {
let timer;
return function(...args) {
clearTimeout(timer);
timer = setTimeout(() => fn.apply(this, args), delay);
};
}
const debouncedFn = debounce(() => console.log('Debounced!'), 300);
debouncedFn();
debouncedFn(); // Will log "Debounced!" only once after 300ms.
Throttling Example:
CODE
function throttle(fn, interval) {
let lastCall = 0;
return function(...args) {
const now = Date.now();
if (now - lastCall >= interval) {
lastCall = now;
fn.apply(this, args);
}
};
}
const throttledFn = throttle(() => console.log('Throttled!'), 1000);
throttledFn();
throttledFn(); // Will log "Throttled!" at most once per 1000ms.
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