🔥 JavaScript Interview Series(18): Top Coding Challenges for JavaScript Interviews

JavaScript interviews often test how well you can think through problems, write clean code, and optimize solutions under pressure. Below are 10 classic coding challenges that appear frequently in JavaScript interviews—complete with explanations, model answers, and realistic follow-up questions to sharpen your interview readiness.

---

1. Reverse a String

Key Concept: String manipulation and array methods.

Model Answer:
You can reverse a string in JavaScript by converting it into an array, reversing it, and joining it back:

function reverseString(str) {
  return str.split('').reverse().join('');
}

Alternatively, using a loop:

function reverseString(str) {
  let reversed = '';
  for (let char of str) reversed = char + reversed;
  return reversed;
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How would you handle Unicode or emoji characters properly?
2. Can you reverse a string without using built-in methods?
3. What is the time complexity of your approach?

---

2. Check for Palindrome

Key Concept: String normalization, comparison logic.

Model Answer:
A palindrome reads the same forward and backward.

function isPalindrome(str) {
  const cleaned = str.toLowerCase().replace(/[^a-z0-9]/g, '');
  return cleaned === cleaned.split('').reverse().join('');
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How would you optimize this for very long strings?
2. Can you check palindrome using recursion?
3. How would you handle case sensitivity or spaces?

---

3. Find the Longest Word in a Sentence

Key Concept: Array iteration, string splitting, and reduce.

Model Answer:

function longestWord(sentence) {
  return sentence.split(' ').reduce((longest, word) =>
    word.length > longest.length ? word : longest
  , '');
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How can you handle punctuation marks efficiently?
2. How would you modify this to return multiple longest words?
3. What’s the complexity of your solution?

---

4. FizzBuzz

Key Concept: Loops, conditionals, and modulo operations.

Model Answer:

for (let i = 1; i <= 100; i++) {
  let output = '';
  if (i % 3 === 0) output += 'Fizz';
  if (i % 5 === 0) output += 'Buzz';
  console.log(output || i);
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How would you write this without using conditionals?
2. Can you make it dynamic for other divisors?
3. What is the runtime complexity?

---

5. Remove Duplicates from an Array

Key Concept: Sets, array filtering, uniqueness.

Model Answer:

function removeDuplicates(arr) {
  return [...new Set(arr)];
}

Alternative with filter():

function removeDuplicates(arr) {
  return arr.filter((item, index) => arr.indexOf(item) === index);
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. What are the trade-offs between Set and filter()?
2. How would you handle arrays with objects?
3. How do you ensure order is preserved?

---

6. Flatten a Nested Array

Key Concept: Recursion, array manipulation, ES6 methods.

Model Answer:

function flatten(arr) {
  return arr.reduce((flat, item) => 
    flat.concat(Array.isArray(item) ? flatten(item) : item), []);
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Can you solve this without recursion?
2. How would you flatten an array up to a specific depth?
3. How does Array.flat() compare?

---

7. Find the Missing Number

Key Concept: Arithmetic operations and arrays.

Model Answer:
Given numbers from 1 to n, find the missing one.

function findMissing(arr) {
  const n = arr.length + 1;
  const expectedSum = (n * (n + 1)) / 2;
  const actualSum = arr.reduce((a, b) => a + b, 0);
  return expectedSum - actualSum;
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. What if there are multiple missing numbers?
2. Can you solve it without using the sum formula?
3. What is the time and space complexity?

---

8. Count Character Frequency

Key Concept: Hash maps and string iteration.

Model Answer:

function charCount(str) {
  const count = {};
  for (let char of str) {
    count[char] = (count[char] || 0) + 1;
  }
  return count;
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How would you handle case-insensitive counting?
2. Can you find the most frequent character efficiently?
3. How does this compare to using Map objects?

---

9. Find the First Non-Repeating Character

Key Concept: Frequency maps, iteration order.

Model Answer:

function firstUniqueChar(str) {
  const count = {};
  for (let char of str) count[char] = (count[char] || 0) + 1;
  for (let char of str) if (count[char] === 1) return char;
  return null;
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How would you handle large inputs efficiently?
2. How can you modify this to return the index instead?
3. Can you use ES6 structures for better performance?

---

10. Implement a Debounce Function

Key Concept: Closures, higher-order functions, asynchronous behavior.

Model Answer:

function debounce(fn, delay) {
  let timer;
  return function(...args) {
    clearTimeout(timer);
    timer = setTimeout(() => fn.apply(this, args), delay);
  };
}

Possible 3 Follow-ups: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. What’s the difference between debounce and throttle?
2. How would you handle immediate execution?
3. Can you implement this using requestAnimationFrame?

---

✨ Final Thoughts

Mastering these challenges helps you demonstrate logical reasoning, clean code structure, and understanding of core JavaScript concepts. Practice them not just to memorize solutions—but to explain your reasoning process clearly in an interview.

Want to experience a real-time AI mock interview with voice feedback? Try it here: 👉 offereasy.ai