Sunny Wilson

Posted on Feb 25

I Built a JavaScript Interview Prep Platform with Interactive Code Visualizations

#javascript #webdev #beginners #career

If you've ever bombed a JavaScript interview because you understood a concept but couldn't explain it clearly — this post is for you.

I've been building jsinterview.dev — a free JavaScript interview prep platform that teaches you concepts through interactive step-by-step code visualizations. Think of it as Python Tutor, but purpose-built for JS interviews.

The Problem I Was Solving

Most JS learning resources fall into one of two camps:

Read-heavy docs — great for reference, terrible for building intuition
LeetCode-style grinders — great for algorithms, but skip the JS internals entirely

When I started prepping for frontend interviews, I kept hitting the same wall. I knew what a closure was. I could recite the definition. But when an interviewer asked me to walk through how the call stack and heap memory behave when a closure forms — I froze.

The missing piece? Visual, step-by-step execution.

What jsinterview.dev Offers

The platform is structured around 4 phases that mirror how real interviews unfold:

1. Understand — What interviewers ask you to explain

JavaScript Deep Dive (88 topics): Closures, Event Loop, Prototypes, this, V8 Engine internals
DSA Fundamentals (20 topics): Big O, Arrays, Hash Tables, Stacks, Queues, Linked Lists

2. Build — What interviewers ask you to implement

JavaScript Core (13 problems): closures, this, prototypes, hoisting, scope
Async JavaScript (3 problems): Promises, async/await, event loop patterns
Array Polyfills (17 problems): implement map, filter, reduce, flat from scratch
Utility Functions (8 problems): debounce, throttle, deep clone, memoize

3. Solve — Algorithm-focused interview rounds

178 DSA problems across 19 topics: Arrays & Hashing, Two Pointers, Sliding Window, Binary Search, Linked List, Sorting, and more

4. Answer — What interviewers ask you to explain verbally

100 questions each for HTML, CSS, JavaScript, React, and Bundlers (Webpack, Vite, Rollup, esbuild)

The Core Feature: Interactive Visualizations

This is the part I'm most proud of. Every concept comes with an interactive code visualization that steps through execution phase by phase.

For example, on the "What is a Closure?" page, you don't just read a definition. You step through:

function outer() {
  let x = 10;
  function inner() {
    return x;
  }
  return inner;
}
const fn = outer();
fn(); // 10

...and watch the Call Stack and Heap Memory update in real-time at each step. You see exactly when the closure forms, what gets captured in memory, and why fn() still returns 10 after outer() has returned.

Each visualization has Beginner / Intermediate / Advanced levels, so you can start simple and work up to edge cases.

The same treatment applies to the Event Loop, prototypes, this binding, and more:

By the Numbers

88 JS concepts
245 coding problems
27 categories
525+ interview questions across HTML, CSS, JS, React, and Bundlers
100% free

Who It's For

Developers prepping for frontend or full-stack interviews
Engineers who want to truly understand JS internals, not just pass tests
Anyone who's been asked "explain the event loop" and wanted to show rather than just tell

Try It

👉 jsinterview.dev

Start with the JavaScript Deep Dive if you want to nail the conceptual questions, or jump straight into the Build section if you want hands-on practice implementing things like debounce, Promise.all, or a custom Array.prototype.map.

Would love any feedback from the dev.to community — especially on topics you wish were covered. Drop a comment below!

DEV Community