JavaScript Notes

Top 30 JavaScript Coding Interview Questions 2026 (With Solutions)

Top 30 JavaScript coding interview questions with step-by-step solutions. Covers arrays, strings, objects, recursion, algorithms, and problem-solving patterns for 2026 interviews.

These coding questions test problem-solving ability, not just JavaScript knowledge. They appear in real interviews at product companies, startups, and big tech.

Section 1: Array Problems

Q1: Flatten a Nested Array

Example

// Implement: flattenArray([[1, [2, [3, [4]]]], 5]) → [1, 2, 3, 4, 5] function flattenArray(arr) { return arr.reduce((acc, item) => { if (Array.isArray(item)) { return acc.concat(flattenArray(item)); } return acc.concat(item); }, []); } console.log(flattenArray([1, [2, [3, [4]]]])); console.log(flattenArray([[1, 2], [3, [4, 5]]]));

Output

[1, 2, 3, 4]
[1, 2, 3, 4, 5]

Alternative using built-in:

Example

[1, [2, [3]]].flat(Infinity); // Native ES2019+

Q2: Remove Duplicates from Array

Example

function unique(arr) { return [...new Set(arr)]; } // Using filter: function uniqueFilter(arr) { return arr.filter((item, index) => arr.indexOf(item) === index); } console.log(unique([1, 2, 2, 3, 3, 4, 1])); console.log(unique(["a", "b", "a", "c"]));

Output

[1, 2, 3, 4]
["a", "b", "c"]

Q3: Chunk Array

Example

function chunk(arr, size) { const result = []; for (let i = 0; i < arr.length; i += size) { result.push(arr.slice(i, i + size)); } return result; } console.log(chunk([1, 2, 3, 4, 5], 2)); console.log(chunk([1, 2, 3, 4, 5], 3));

Output

[[1, 2], [3, 4], [5]]
[[1, 2, 3], [4, 5]]

Q4: Two Sum

Example

Output

[0, 1]
[1, 2]

Time: O(n) | Space: O(n)

Q5: Group By Key

Example

Output

{
  Delhi: [{ name: "Alice", city: "Delhi" }, { name: "Carol", city: "Delhi" }],
  Mumbai: [{ name: "Bob", city: "Mumbai" }]
}

Section 2: String Problems

Q6: Check Palindrome

Example

function isPalindrome(str) { const clean = str.toLowerCase().replace(/[^a-z0-9]/g, ""); return clean === clean.split("").reverse().join(""); } console.log(isPalindrome("racecar")); // true console.log(isPalindrome("A man a plan a canal Panama")); // true console.log(isPalindrome("hello")); // false

Output

true
true
false

Q7: Check Anagram

Example

Output

true
false

Q8: Count Character Frequency

Example

function charFrequency(str) { return str.split("").reduce((acc, char) => { acc[char] = (acc[char] || 0) + 1; return acc; }, {}); } console.log(charFrequency("javascript"));

Output

{ j: 1, a: 2, v: 1, s: 1, c: 1, r: 1, i: 1, p: 1, t: 1 }

Q9: Capitalize Words

Example

function titleCase(str) { return str .split(" ") .map(word => word.charAt(0).toUpperCase() + word.slice(1).toLowerCase()) .join(" "); } console.log(titleCase("hello world from javascript"));

Output

Hello World From Javascript

Q10: Find Longest Word

Example

function longestWord(sentence) { return sentence .split(" ") .reduce((longest, word) => word.length > longest.length ? word : longest, ""); } console.log(longestWord("The quick brown fox jumped"));

Output

jumped

Section 3: Object Problems

Q11: Deep Clone an Object

Example

Output

2
99

Q12: Flatten a Nested Object

Example

Output

{ "a.b.c": 1, "a.d": 2, e: 3 }

Q13: Object to Query String

Example

function toQueryString(params) { return Object.entries(params) .map(([key, val]) => `${encodeURIComponent(key)}=${encodeURIComponent(val)}`) .join("&"); } console.log(toQueryString({ name: "Alice", age: 25, city: "New Delhi" }));

Output

name=Alice&age=25&city=New%20Delhi

Section 4: Function Problems

Q14: Custom map()

Example

Array.prototype.myMap = function (callback) { const result = []; for (let i = 0; i < this.length; i++) { result.push(callback(this[i], i, this)); } return result; }; console.log([1, 2, 3].myMap(x => x * 2)); console.log([1, 2, 3].myMap((x, i) => `${i}:${x}`));

Output

[2, 4, 6]
["0:1", "1:2", "2:3"]

Q15: Custom filter()

Example

Array.prototype.myFilter = function (callback) { const result = []; for (let i = 0; i < this.length; i++) { if (callback(this[i], i, this)) { result.push(this[i]); } } return result; }; console.log([1, 2, 3, 4, 5].myFilter(x => x % 2 === 0));

Output

[2, 4]

Q16: Custom reduce()

Example

Array.prototype.myReduce = function (callback, initialValue) { let acc = initialValue !== undefined ? initialValue : this[0]; const start = initialValue !== undefined ? 0 : 1; for (let i = start; i < this.length; i++) { acc = callback(acc, this[i], i, this); } return acc; }; console.log([1, 2, 3, 4].myReduce((acc, val) => acc + val, 0)); console.log([1, 2, 3, 4].myReduce((acc, val) => acc * val));

Output

10
24

Q17: Debounce Function

Example

Q18: Throttle Function

Example

Q19: Function Composition

Example

const compose = (...fns) => (x) => fns.reduceRight((acc, fn) => fn(acc), x); const pipe = (...fns) => (x) => fns.reduce((acc, fn) => fn(acc), x); const double = x => x * 2; const addOne = x => x + 1; const square = x => x * x; const transform = compose(square, addOne, double); console.log(transform(3)); // square(addOne(double(3))) = square(7) = 49 const pipeline = pipe(double, addOne, square); console.log(pipeline(3)); // square(addOne(double(3))) = same

Output

49
49

Q20: Curry Function

Example

Output

6
6
6

Section 5: Recursion Problems

Q21: Fibonacci (Recursive + Memoized)

Example

// Recursive (exponential) function fib(n) { if (n <= 1) return n; return fib(n - 1) + fib(n - 2); } // Memoized (O(n)) function fibMemo(n, memo = {}) { if (n in memo) return memo[n]; if (n <= 1) return n; memo[n] = fibMemo(n - 1, memo) + fibMemo(n - 2, memo); return memo[n]; } console.log(fib(10)); console.log(fibMemo(50));

Output

55
12586269025

Q22: Deep Equal

Example

Output

true
false

Q23: Power Function

js example

function power(base, exp) {
  if (exp === 0) return 1;
  if (exp % 2 === 0) {
    const half = power(base, exp / 2);
    return half * half;
  }
  return base * power(base, exp - 1);
}

console.log(power(2, 10)); // 1024
console.log(power(3, 4));  // 81

Output

1024
81

Section 6: Design Pattern Problems

Q24: Implement EventEmitter

Example

class EventEmitter { constructor() { this.events = {}; } on(event, listener) { this.events[event] = this.events[event] || []; this.events[event].push(listener); return this; } emit(event, ...args) { (this.events[event] || []).forEach(fn => fn(...args)); return this; } off(event, listener) { this.events[event] = (this.events[event] || []).filter(fn => fn !== listener); return this; } once(event, listener) { const wrapper = (...args) => { listener(...args); this.off(event, wrapper); }; return this.on(event, wrapper); } } const emitter = new EventEmitter(); emitter.on("data", d => console.log("Received:", d)); emitter.emit("data", "Hello!"); emitter.emit("data", "World!");

Output

Received: Hello!
Received: World!

Q25: LRU Cache

js example

class LRUCache {
  constructor(capacity) {
    this.capacity = capacity;
    this.cache = new Map();
  }

  get(key) {
    if (!this.cache.has(key)) return -1;
    const val = this.cache.get(key);
    this.cache.delete(key);
    this.cache.set(key, val); // Move to end (most recent)
    return val;
  }

  put(key, val) {
    if (this.cache.has(key)) this.cache.delete(key);
    this.cache.set(key, val);
    if (this.cache.size > this.capacity) {
      this.cache.delete(this.cache.keys().next().value); // Remove oldest
    }
  }
}

const cache = new LRUCache(3);
cache.put(1, "A");
cache.put(2, "B");
cache.put(3, "C");
console.log(cache.get(1));   // A (1 is now most recent)
cache.put(4, "D");           // evicts 2 (least recently used)
console.log(cache.get(2));   // -1 (evicted)
console.log(cache.get(3));   // C

Output

A
-1
C

Common Traps 🪤

Mistake	Fix
Using `==` instead of `===`	Always use strict equality
Mutating input array	Use `[...arr]` or `arr.slice()` to copy
Off-by-one in loops	Test with small arrays manually
Not handling empty input	Add guard `if (!arr	!arr.length) return`
Forgetting to return in reduce	Always verify your accumulator logic

Time Complexity Quick Reference

Solution	Time	Space
Two Sum (hash map)	O(n)	O(n)
Flatten (recursive)	O(n)	O(d) — d=depth
Deep Clone	O(n)	O(n)
Debounce/Throttle	O(1)	O(1)
Fibonacci (memo)	O(n)	O(n)
LRU Cache get/put	O(1)	O(n)

Continue the lesson path

Study next in this module

4 connected lessons

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