Free JavaScript interview practice and DSA

For full-stack developers.

Pick your role and interview date. Each day: what’s due, your weak spots, then something new.

I'm adeveloper

1 · You

A full-stack developer. Interview in 12 days, 30 minutes a day.

2 · Today’s plan

8 reviewsClosures, Indexes, Effects and 5 more
1 lessonTwo pointers
2 problemsValid Palindrome, debounce
3 newNode.js › Streams & backpressure

3 · Back before you forget

Closuresin 7 days
Indexestomorrow
Two pointersin 3 days

An example plan.

Prep fades. Spaced repetition brings it back.

CodePark schedules it. Got it? Back in 1, 3, 7, 14, 30, then 60 days. Missed it? Back tomorrow.

Read once Reviewed with spaced repetitionAn illustration of the forgetting curve, not measured data.

Weak at DSA?

DSA for beginners.

18 beginner lessons in plain words, one per pattern: the idea, a template, a worked example. Press Play.

Watch it run. Each pattern has a visual you step through, and it stops to ask what happens next.
Plan before you code. Name the pattern and how you'll use it, then write the code.
Hints that don't spoil it. The pattern first, then the approach, then a skeleton, then nearly there.
Beat the clock. Every problem has a target time, like a real interview.

Read the first lessonThen 62 DSA problems, grouped by pattern.

Try it here.

The real question views and editor: an open question, a multiple choice, a coding problem. Nothing is saved.

Example · nothing is saved

loading the demo…

Frontend, React and Node.js interview questions.

From 1,617, each with a model answer and the key points.

Frontend · JavaScript

Explain hoisting and the temporal dead zone (TDZ).

Frontend · JavaScript

What is a closure? Give a practical example.

Frontend · React

What is the difference between props and state?

Frontend · React

What are the rules of hooks, and why do they exist?

Frontend · Next.js

What does Next.js add on top of plain React?

Frontend · Next.js

How do you fetch data in the App Router?

Frontend · Testing

How do you test a component that shows a loading state, then fetched data?

Frontend · Testing

What's the difference between a mock, a stub, a spy and a fake?

Frontend · TypeScript

What is narrowing? List the main ways to narrow a union type.

Frontend · TypeScript

Name the built-in utility types you use most and what each one does.

Frontend · CSS & HTML

What is semantic HTML and why does it matter? Give examples of semantic elements.

Frontend · CSS & HTML

Explain the main axis and cross axis in Flexbox. Which properties align along each?

Backend · Node.js

Is Node.js single-threaded? How does it handle many concurrent requests?

Backend · Node.js

Describe the phases of the Node.js event loop.

Backend · NestJS

What is NestJS, and why would a team pick it over plain Express?

Backend · NestJS

What is a guard in Nest, and why is it a better place for authorization than middleware?

Backend · Docker

What is the difference between a container and a virtual machine?

Backend · Docker

How does the Docker build cache work, and what invalidates it?

Backend · Redis & caching

Why is Redis so fast, and is it really single-threaded?

Backend · Redis & caching

Name the main Redis data types and give a typical use case for each.

Backend · Databases & SQL

What is a database index, and how does a B-tree index find rows?

Backend · Databases & SQL

What does ACID stand for? Explain each property with a money transfer example.

Backend · HTTP & API design

What are the core principles of REST?

Backend · HTTP & API design

Compare offset pagination and cursor pagination for an API. When would you choose each?

A full-stack plan uses all three. Switch any on later.

Frontend

619 questions in 11 topics

JavaScript, TypeScript, React and Next.js
CSS & HTML, accessibility and internationalization
Browser & web performance, testing and security
Frontend system design

Backend

998 questions in 16 topics

Node.js, NestJS, JavaScript and TypeScript
Databases & SQL, Redis & caching, message queues
HTTP & API design, auth & security
System design, Docker, AWS and Linux

Coding

103 problems and 18 lessons

34 JavaScript problems, like debounce, Promise.all and deep clone
62 DSA problems, grouped by pattern
7 class design problems, in parts
18 beginner lessons

Built for practice.

Progress and weak spots.

Your weakest topics come first.

Progress: strength by topic, with the weakest topics first

An example: thirty weeks of practice.

Review on your phone.

Lessons and reviews fit a phone.

A question on a phone, with the key points and rating buttons

Works offline.

Install it from your browser.

Your editor, your colors.

Light, or VS Code’s with TypeScript.

Backups.

Download your progress any time.

Why I built CodePark

I’m a frontend and full-stack developer, and DSA was always my weak spot. I’d solve a problem, feel good about it, and three weeks later look at the same problem with no idea where to start.

Interview questions went the same way. I’d cram them before an interview and they’d be gone a month later. The problem wasn’t learning. It was keeping it.

So I built what I wanted: one plan a day, lessons that start from zero, and reviews that bring things back just before they fade. It’s free, and it will stay free.

Omar · built CodePark

Your progress is yours.

As a guest, your progress stays in your browser.
Accounts are optional: sync with GitHub or Google, delete any time.
No ads, and no ad cookies.
Usage reports are anonymous, and off with Do Not Track.

Read exactly what's kept →

Questions, answered.

Is it free?

Yes. All questions, problems and lessons are free and stay free. Features marked “Pro, free during beta” are free for now and will become part of Pro later, which also adds AI feedback.

Who is it for?

Developers getting ready for frontend, backend or full-stack interviews in JavaScript or TypeScript. The DSA lessons start from zero.

Does it cover React, Node.js and TypeScript?

Yes. Frontend covers JavaScript, React, Next.js, TypeScript, CSS and HTML, browser performance and frontend system design. Backend covers Node.js, NestJS, databases and SQL, API design, Redis, Docker and system design.

Can I practice LeetCode-style problems in JavaScript?

Yes. The DSA problems are grouped by pattern, like two pointers and sliding window, and each pattern has a beginner lesson. There are also JavaScript utilities, like debounce and throttle, as asked in machine coding rounds. You write the code in the browser and the tests run there.

How does spaced repetition help with coding interviews?

You forget most of what you practice within a few weeks. CodePark brings each question and problem back just before you would forget it: after 1, 3, 7, 14, 30, then 60 days. Miss it, and it comes back tomorrow.

My interview is in two weeks. Is that enough?

Set the date and CodePark fits each day to the time left. It starts with the must-know questions and your weakest topics.

Do I need an account?

No. Start as a guest and your progress stays in this browser. Sign in with GitHub or Google to sync it across devices.

Can I use it on my phone?

Yes, for lessons, questions and reviews. Coding works best on a bigger screen.

Does it work offline?

Yes. Install it from your browser and it keeps working without a connection.

Which languages?

JavaScript and TypeScript. The tests run in your browser.

Where is my data?

As a guest, only in your browser. Signed in, on CodePark's server, where you can download or delete it at any time.

Learn it once.
Know it on interview day.

Free, with no sign-up.

Start practicing Sign in to sync

Interview practice that sticks.