JavaScript Interview Questions

Junior JavaScript Developer Interview Questions

Senior JavaScript Developer Interview Questions

JavaScript Architect / Frontend Architect Interview Questions

30 JavaScript Coding Interview Questions with Solutions

1. Reverse a String

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

console.log(reverseString("hello"));

2. Check Palindrome

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

3. Find Largest Number

function findLargest(numbers) {
  return Math.max(...numbers);
}

4. Remove Duplicates

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

5. Count Vowels

function countVowels(str) {
  return str.match(/[aeiou]/gi)?.length || 0;
}

6. FizzBuzz

function fizzBuzz(n) {
  for (let i = 1; i <= n; i++) {
    if (i % 15 === 0) console.log("FizzBuzz");
    else if (i % 3 === 0) console.log("Fizz");
    else if (i % 5 === 0) console.log("Buzz");
    else console.log(i);
  }
}

7. Factorial

function factorial(n) {
  if (n === 0) return 1;
  return n * factorial(n - 1);
}

8. Fibonacci

function fibonacci(n) {
  const result = [0, 1];

  for (let i = 2; i < n; i++) {
    result.push(result[i - 1] + result[i - 2]);
  }

  return result.slice(0, n);
}

9. Flatten Array

function flattenArray(arr) {
  return arr.flat(Infinity);
}

10. Deep Clone Object

function deepClone(value) {
  return structuredClone(value);
}

11. Debounce

function debounce(fn, delay) {
  let timer;

  return function(...args) {
    clearTimeout(timer);

    timer = setTimeout(function() {
      fn.apply(this, args);
    }, delay);
  };
}

12. Throttle

function throttle(fn, delay) {
  let lastTime = 0;

  return function(...args) {
    const now = Date.now();

    if (now - lastTime >= delay) {
      lastTime = now;
      fn.apply(this, args);
    }
  };
}

13. Memoization

function memoize(fn) {
  const cache = {};

  return function(value) {
    if (cache[value]) return cache[value];

    cache[value] = fn(value);
    return cache[value];
  };
}

14. Curry Function

function curryAdd(a) {
  return function(b) {
    return a + b;
  };
}

15. Group Array by Key

function groupBy(items, key) {
  return items.reduce(function(result, item) {
    const group = item[key];

    if (!result[group]) {
      result[group] = [];
    }

    result[group].push(item);
    return result;
  }, {});
}

16. Custom map()

Array.prototype.customMap = function(callback) {
  const result = [];

  for (let i = 0; i < this.length; i++) {
    result.push(callback(this[i], i, this));
  }

  return result;
};

17. Custom filter()

Array.prototype.customFilter = function(callback) {
  const result = [];

  for (let i = 0; i < this.length; i++) {
    if (callback(this[i], i, this)) {
      result.push(this[i]);
    }
  }

  return result;
};

18. Custom reduce()

Array.prototype.customReduce = function(callback, initialValue) {
  let accumulator = initialValue;

  for (let i = 0; i < this.length; i++) {
    accumulator = callback(accumulator, this[i], i, this);
  }

  return accumulator;
};

19. Promise.all Polyfill

function promiseAll(promises) {
  return new Promise(function(resolve, reject) {
    const results = [];
    let completed = 0;

    promises.forEach(function(promise, index) {
      Promise.resolve(promise)
        .then(function(value) {
          results[index] = value;
          completed++;

          if (completed === promises.length) {
            resolve(results);
          }
        })
        .catch(reject);
    });
  });
}

20. Event Emitter

class EventEmitter {
  constructor() {
    this.events = {};
  }

  on(event, callback) {
    if (!this.events[event]) {
      this.events[event] = [];
    }

    this.events[event].push(callback);
  }

  emit(event, data) {
    if (this.events[event]) {
      this.events[event].forEach(function(callback) {
        callback(data);
      });
    }
  }
}

21. Check Anagram

function isAnagram(a, b) {
  return a.split("").sort().join("") === b.split("").sort().join("");
}

22. Capitalize Words

function capitalizeWords(str) {
  return str
    .split(" ")
    .map(function(word) {
      return word.charAt(0).toUpperCase() + word.slice(1);
    })
    .join(" ");
}

23. Find Missing Number

function findMissingNumber(arr, n) {
  const expected = (n * (n + 1)) / 2;
  const actual = arr.reduce((sum, num) => sum + num, 0);

  return expected - actual;
}

24. Count Occurrences

function countOccurrences(arr) {
  return arr.reduce(function(result, item) {
    result[item] = (result[item] || 0) + 1;
    return result;
  }, {});
}

25. Merge Objects

function mergeObjects(a, b) {
  return { ...a, ...b };
}

26. Safe JSON Parse

function safeJsonParse(text) {
  try {
    return JSON.parse(text);
  } catch (error) {
    return null;
  }
}

27. Retry Async Function

async function retry(fn, attempts) {
  for (let i = 0; i < attempts; i++) {
    try {
      return await fn();
    } catch (error) {
      if (i === attempts - 1) throw error;
    }
  }
}

28. Timeout Promise

function delay(ms) {
  return new Promise(function(resolve) {
    setTimeout(resolve, ms);
  });
}

29. Toggle Class

function toggleActive(element) {
  element.classList.toggle("active");
}

30. Validate Email

function validateEmail(email) {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
}

JavaScript Interview Questions FAQ

How many JavaScript questions should I prepare?

Prepare core theory, coding, debugging, async JavaScript, DOM, browser APIs, performance, security, and real-world scenario questions.

What should junior developers focus on?

Variables, data types, functions, arrays, objects, DOM, events, JSON, fetch, promises, and basic coding problems.

What should senior developers focus on?

Closures, prototypes, event loop, memory leaks, performance, security, testing, async patterns, design patterns, and debugging.

What should architects focus on?

System design, frontend architecture, micro frontends, design systems, scalability, accessibility, performance budgets, governance, and observability.

Are coding questions important?

Yes. Coding questions test problem-solving, JavaScript fundamentals, debugging, edge cases, and clean code.

Conclusion

This JavaScript interview guide covers junior, senior, and architect-level questions with answers, coding examples, debugging topics, async JavaScript, DOM, browser APIs, performance, security, accessibility, modules, closures, prototypes, promises, async/await, and design patterns.

To prepare effectively, practice small examples, explain concepts in your own words, debug real code, and build projects that use APIs, forms, DOM updates, async workflows, and reusable JavaScript patterns.