JavaScript: practice, test and prove it

JavaScript core: scope and hoisting, equality, closures, this, array methods, promises and the event loop.

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JavaScript screenings lean heavily on behaviour that surprises people coming from other languages. Typical questions ask what a snippet logs when var is hoisted, why == and === give different results, what a closure captures inside a loop, and in what order promises and timers run.

Use this page to practise reading code the way the engine runs it. The lessons explain scope and closures first, then asynchronous code and the event loop, with the traps that commonly appear in timed tests.

Benchmark: The pass mark on this site is 80%, and JavaScript is commonly screened for front-end, full-stack and Node.js developer roles. Requirements vary by employer.

Lessons

Scope, hoisting and closures in JavaScriptHow var, let and const differ, what hoisting really does, and why closures inside loops behave as they do.Lesson 1

Promises, async/await and the event loopWhy asynchronous code logs in the order it does, and how to write and handle promises and async functions safely.Lesson 2

Equality, type coercion and what this points toPredict == versus ===, truthy and falsy values, the ?? operator, and why a method loses this when you pass it around.Lesson 3

Array methods, spread and destructuringChain map, filter and reduce correctly, sort numbers without surprises, and copy arrays and objects instead of sharing them.Lesson 4

Interview prep

JavaScript interview questionsFrequently asked JavaScript questions, answered the way an experienced developer would explain them in a technical round.6 questions

Certificate

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Common questions

Should I learn var if modern code uses let and const?

Yes, enough to read it. Older code and many test questions still use var, and its function scope and hoisting explain several classic output puzzles.

Do I need a framework for these questions?

No. This test covers core JavaScript that every framework is built on, and strong fundamentals make learning any framework much faster.

Can I practise without installing anything?

Yes. Every modern browser has a developer console where you can paste a snippet and see the output immediately. Node.js is useful later for running scripts outside the browser.

JavaScript test for interviewWhat the screening round looks like, with a 5-question sample.