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If you’re preparing for frontend or full-stack interviews, mastering JavaScript is essential. Below is a structured list of 50 coding questions frequently asked in interviews, from junior to senior roles.
1. Core Concepts
➢ Explain closures with a counter example.
➢ Implement a custom `bind()` method.
➢ What is hoisting? Demonstrate with var, let, and const.
➢ Difference between `==` and `===`.
➢ Implement currying for any function.
➢ Deep vs shallow copy – implement both.
➢ What are IIFEs and why are they used?
➢ Debounce vs Throttle – and implement both.
➢ Event loop simulation question.
➢ Use `call`, `apply`, and `bind` with examples.
2. Arrays & Objects
➢ Reverse an array in-place.
➢ Remove duplicates from an array.
➢ Flatten a nested array.
➢ Rotate array by `k` positions.
➢ Find second largest element.
➢ Merge two sorted arrays.
➢ Intersection of two arrays.
➢ Group anagrams from a list.
➢ Deep clone an object.
➢ Count character frequency.
3. String Manipulation
➢ Check if a string is a palindrome.
➢ Find the first non-repeating character.
➢ Longest common prefix.
➢ Reverse words in a sentence.
➢ Check if one string is a rotation of another.
4. Functions & Execution
➢ Memoize any pure function.
➢ Implement a custom `Promise.all()`.
➢ Retry API with exponential backoff.
➢ Limit concurrent promises.
➢ Custom implementation of `map`, `reduce`, `filter`.
5. Algorithms
➢ Kadane’s Algorithm for max subarray sum.
➢ Sliding window to find max sum of size k.
➢ Two pointer technique for pair sum.
➢ In-place sort of 0s, 1s, and 2s.
➢ Move all 0s to the end of array.
➢ Longest substring without repeating characters.
➢ Valid parentheses using a stack.
➢ LRU Cache using `Map`.
➢ Implement a basic Trie structure.
➢ Binary search in a sorted array.
6. Async & Promises
➢ Chaining multiple promises.
➢ Explain microtask vs macrotask.
➢ Convert callback-based API to promise.
➢ Fetch with retry on failure.
➢ Sleep function using `Promise`.
7. Practical/Interview-Favorite
➢ Infinite scroll logic.
➢ Simple pagination function.
➢ Autocomplete with debounce.
➢ Module pattern using IIFE.
➢ Serialize/deserialize a DOM tree.
✅ Bonus Tips:
- Don’t just read — implement.
- Explain your code out loud (this improves clarity).
- Always talk about time and space complexity.
- Interviewers value clarity > cleverness.
You Should Know:
JavaScript Code Examples
1. Closures Example
function outer() {
let count = 0;
return function inner() {
count++;
return count;
};
}
const counter = outer();
console.log(counter()); // 1
console.log(counter()); // 2
2. Custom `bind()` Implementation
Function.prototype.customBind = function(context, ...args) {
const fn = this;
return function(...newArgs) {
return fn.apply(context, [...args, ...newArgs]);
};
};
3. Debounce Implementation
function debounce(func, delay) {
let timeout;
return function(...args) {
clearTimeout(timeout);
timeout = setTimeout(() => func.apply(this, args), delay);
};
}
4. Deep Copy vs Shallow Copy
// Shallow Copy
const obj = { a: 1, b: { c: 2 } };
const shallowCopy = { ...obj };
// Deep Copy
const deepCopy = JSON.parse(JSON.stringify(obj));
5. Binary Search
function binarySearch(arr, target) {
let left = 0, right = arr.length - 1;
while (left <= right) {
const mid = Math.floor((left + right) / 2);
if (arr[bash] === target) return mid;
if (arr[bash] < target) left = mid + 1;
else right = mid - 1;
}
return -1;
}
What Undercode Say:
Mastering these JavaScript concepts is crucial for technical interviews. Practice implementing each question, optimize for performance, and understand the underlying mechanisms (e.g., event loop, closures, and prototypal inheritance).
Linux/IT Commands Related to JavaScript Development:
Monitor Node.js processes ps aux | grep node Kill a Node process kill -9 $(lsof -t -i:3000) Check memory usage node --inspect-brk script.js Debug Node.js with Chrome DevTools node --inspect script.js Install dependencies npm install --save-dev eslint prettier Run security audit npm audit Benchmark JavaScript performance time node script.js
Windows Commands for Developers:
:: List running processes tasklist | findstr node :: Kill a process taskkill /F /PID <ProcessID> :: Check network connections netstat -ano | findstr 3000
Expected Output:
A well-prepared JavaScript developer should be able to:
✅ Solve algorithmic problems efficiently.
✅ Explain core JavaScript concepts clearly.
✅ Debug and optimize code.
✅ Work with asynchronous operations (Promises, async/await).
✅ Implement design patterns (Module, Factory, Observer).
Prediction:
JavaScript will continue dominating frontend development, with increasing demand for full-stack developers who understand both client-side and server-side (Node.js) JavaScript. Mastering these concepts ensures long-term career growth.
🔗 Useful Resources:
References:
Reported By: Akashsinnghh 50 – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
