JavaScript
Build instincts for runtime behavior, references, closures, promises, and async failures.
Practice JavaScript, TypeScript, React, Node.js, and DSA in one hands-on environment. Start free, then use guided feedback to turn your practice into a focused interview-readiness path.
Free to start · No credit card required · No fixed schedule
Challenge 03
Practice a focused algorithm problem, verify edge cases, and review the quality of your approach.
def sum_two_numbers(a, b):
return a + b
# Try a few inputs
result = sum_two_numbers(4, 7)
Run the visible examples to check your solution.
Your code quality review appears after the tests pass.
Choose a track and try a lightweight version of the Codstak workflow.
Five ways to practice
Start wherever you need practice. When you are ready for more direction, AI Coach connects that work to the skills you need to demonstrate in an interview.
See the frontend interview prep pathBuild instincts for runtime behavior, references, closures, promises, and async failures.
Learn type annotations, objects, unions, and safe everyday patterns step by step.
Build interview-style interfaces with state, effects, forms, accessibility, and feedback.
Practice HTTP contracts, validation, authorization, webhooks, and resilient API behavior.
Strengthen your core data structures and algorithmic thinking through focused exercises.
How it works
No long course to finish before you touch the code. Pick a skill and start working.
Work on one practical concept at a time, from data structures and runtime reasoning to types, UI behavior, and API contracts.
Write code in the browser, inspect the live result, and keep your full attention on the problem.
Run test cases, review feedback, and use the AI coach to understand what the work demonstrates and what to practise next.
Guided interview readiness
Ask for a hint, understand an error, or get your solution reviewed. The coach helps you reason through the next step so you can build skill evidence—not just collect completed challenges.
Codstak Coach
Ready for this challenge
Free practice, guided progression
Free
An open practice library for building fundamentals, exploring tracks, and working at your own pace.
Start practicingAI Coach
Early accessProgressive guidance across every practice track.
Join early accessAI hints estimator
Pick your practice style and see how your 3-month allowance holds up.
When I'm genuinely stuck
Hints over 3 months
144
AI Coach gives you plenty of room at this pace.
That's 144 hints — the 3-month AI Coach plan includes 1,200.
AI Coach early access is built around guided progression and skill evidence. It does not guarantee an interview outcome, and joining does not start a subscription or require a credit card.
What comes next
Free practice gives you room to build. Guided practice adds targeted feedback and a clearer view of the skills you are demonstrating as you prepare for frontend interviews.
Build the habit
Join the early-access list for product updates and the $9 AI Coach launch.
Read practical lessons on the Codstak newsletter