Intermediate Level JavaScript Interview Questions and Answers
1. What is the difference between == and ===?
Answer:
==performs loose equality with type coercion.===performs strict equality without type coercion.
Example:
console.log(5 == "5"); // true
console.log(5 === "5"); // false
2. What is the ternary operator, and how is it used?
Answer: The ternary operator (?:) is a shorthand for if-else, evaluating a condition and returning one of two values.
Example:
let age = 20;
let status = age >= 18 ? "Adult" : "Minor";
console.log(status); // "Adult"
3. What are string templates (template literals)?
Answer: Template literals, enclosed in backticks (``), allow embedded expressions and multi-line strings.
Example:
let name = "Alice";
console.log(`Hello, ${name}!`); // "Hello, Alice!"
4. What is the BigInt data type?
Answer: BigInt is a primitive type for integers larger than Number.MAX_SAFE_INTEGER (2^53 - 1).
Example:
let big = 123456789012345678901234567890n;
console.log(big + 1n); // 123456789012345678901234567891n
5. How does the sort() method work for arrays?
Answer: Sorts array elements in place. A comparison function can define custom order.
Example:
let arr = [3, 1, 2];
arr.sort((a, b) => a - b); // [1, 2, 3]
6. What are array iteration methods?
Answer: Methods like forEach(), map(), filter(), and reduce() iterate over arrays to process elements.
Example:
let arr = [1, 2, 3];
let doubled = arr.map(x => x * 2); // [2, 4, 6]
7. What is the Date object, and how do you get the current date?
Answer: Represents dates and times. Create a new Date instance to get the current date.
Example:
let now = new Date();
console.log(now); // e.g., 2025-10-18T09:04:00.000Z
8. Difference between for...in and for...of loops
Answer:
for...in: Iterates over enumerable properties of an object.for...of: Iterates over iterable values like arrays or strings.
Example:
let arr = [1, 2, 3];
for (let i in arr) console.log(i); // "0", "1", "2"
for (let v of arr) console.log(v); // 1, 2, 3
9. What are arrow functions, and how do they differ from regular functions?
Answer: Arrow functions (=>) have concise syntax and lexical this binding.
Example:
const add = (a, b) => a + b;
console.log(add(2, 3)); // 5
10. What is the this keyword in JavaScript?
Answer: Refers to the context in which a function is called. It varies based on invocation (object, global, or undefined in strict mode).
Example:
const obj = {
name: "John",
greet() { console.log(this.name); }
};
obj.greet(); // "John"
11. What are closures in JavaScript?
Answer: A closure is a function that retains access to its outer scope’s variables even after the outer function has finished executing.
Example:
function outer() {
let count = 0;
return function inner() {
return count++;
};
}
let counter = outer();
console.log(counter()); // 0
console.log(counter()); // 1
12. What is object destructuring?
Answer: Extracts properties from objects into variables.
Example:
let person = { name: "Alice", age: 25 };
let { name, age } = person;
console.log(name, age); // "Alice", 25
13. What is the purpose of Object.freeze()?
Answer: Prevents modifications to an object, making it immutable.
Example:
let obj = Object.freeze({ name: "John" });
obj.name = "Jane"; // Ignored
console.log(obj.name); // "John"
14. What are JavaScript classes, and how do they work?
Answer: Classes are blueprints for creating objects with shared properties and methods, supporting inheritance via extends.
Example:
class Person {
constructor(name) { this.name = name; }
greet() { return `Hello, ${this.name}`; }
}
let p = new Person("Alice");
console.log(p.greet()); // "Hello, Alice"
15. What is a Set in JavaScript?
Answer: A Set is a collection of unique values with methods like add(), has(), and delete().
Example:
let set = new Set([1, 2, 2, 3]);
console.log(set); // Set {1, 2, 3}
16. What is a Map in JavaScript?
Answer: A Map is a key-value collection where keys can be any type.
Example:
let map = new Map();
map.set("key", "value");
console.log(map.get("key")); // "value"
17. What are iterators in JavaScript?
Answer: Iterators are objects with a next() method returning { value, done } for custom iteration.
Example:
let arr = [1, 2];
let it = arr[Symbol.iterator]();
console.log(it.next()); // { value: 1, done: false }
18. What are regular expressions (RegExp) in JavaScript?
Answer: Patterns used for matching, searching, or replacing strings.
Example:
let str = "Hello123";
console.log(/\\d+/.test(str)); // true
19. What are callbacks in JavaScript?
Answer: Functions passed as arguments to execute after an operation completes.
Example:
setTimeout(() => console.log("Delayed"), 1000);
20. What is event handling in JavaScript?
Answer: Responding to user actions using event listeners like addEventListener.
Example:
document.getElementById("btn").addEventListener("click", () => {
console.log("Button clicked");
});
21. What is event propagation?
Answer: Events travel through the DOM via bubbling (child → parent) or capturing (parent → child).
Example:
document.getElementById("child").addEventListener("click", () => {
console.log("Child clicked");
}, true); // Capture phase
22. What is try...catch used for?
Answer: Handles errors gracefully by catching exceptions.
Example:
try {
throw new Error("Something went wrong");
} catch (e) {
console.log(e.message); // "Something went wrong"
}
23. What is strict mode in JavaScript?
Answer: "use strict" enforces stricter parsing and prevents unsafe actions like undeclared variables.
Example:
"use strict";
x = 10; // ReferenceError
24. What is the DOM, and how do you access elements?
Answer: The Document Object Model represents HTML as a tree of objects. Access elements via getElementById, querySelector, etc.
Example:
let elem = document.querySelector(".myClass");
elem.textContent = "Updated";
25. What is the Window object in the Browser Object Model (BOM)?
Answer: Represents the browser window, providing methods like alert() and setTimeout().
Example:
window.alert("Hello!");
26. What is localStorage in JavaScript?
Answer: Web API for storing persistent key-value pairs in the browser.
Example:
localStorage.setItem("name", "John");
console.log(localStorage.getItem("name")); // "John"
27. What is AJAX, and how does it work?
Answer: AJAX allows asynchronous data fetching without reloading the page. Can use XMLHttpRequest or fetch().
Example:
fetch("<https://api.example.com/data>")
.then(res => res.json())
.then(data => console.log(data));
28. What is JSON, and how do you parse it?
Answer: JSON is a lightweight data format. Use JSON.parse() to convert JSON strings to objects, JSON.stringify() to convert objects to strings.
Example:
let json = '{"name": "John"}';
let obj = JSON.parse(json);
console.log(obj.name); // "John"
29. Difference between call() and apply()
Answer: Both set this context.
call(): Arguments individuallyapply(): Arguments as array
Example:
function greet(greeting) {
console.log(`${greeting}, ${this.name}`);
}
let obj = { name: "Alice" };
greet.call(obj, "Hello"); // "Hello, Alice"
greet.apply(obj, ["Hi"]); // "Hi, Alice"
30. What are higher-order functions?
Answer: Functions that take other functions as arguments or return functions.
Example:
function higher(func) {
return func();
}
higher(() => "Hello!"); // "Hello!"