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Fundamentals

JavaScript Fundamentals Test

20 multiple choice questions on core JavaScript: variables, types, arrays, closures, promises and the event loop.

Questions
20
Time
20 min
Pass mark
60%

No account or email needed.

About this test

This test checks the parts of JavaScript that come up in almost every web developer interview: how values and types behave, how arrays and objects work, how closures and this work, and the order in which asynchronous code runs.

Some questions show a short snippet and ask what it prints. Read the code carefully, because many of them are built around a well-known quirk of the language.

What it covers

The question set has 24 questions: 9 easy, 13 medium and 2 hard. Each attempt draws 20 of them at random, so a second try is not the same test.

  • JavaScript (24)

Try a few sample questions

These are real questions from the set, with the answers and a short explanation. Read the question, decide, then open it to check.

Sample 1 · JavaScript · easyWhat does typeof null return in JavaScript?Show the answer
  • "null"
  • "object"Correct
  • "undefined"
  • "number"

This is a long-standing quirk of the language. typeof null is "object", so check for null with value === null instead.

Sample 2 · JavaScript · easyWhich keyword declares a block-scoped variable that can be reassigned?Show the answer
  • var
  • letCorrect
  • const
  • function

let is block-scoped and can be reassigned. const is block-scoped but cannot be reassigned. var is function-scoped.

Sample 3 · JavaScript · easyWhat does this code print?Show the answer
javascript
console.log(0.1 + 0.2 === 0.3);
  • true
  • falseCorrect
  • undefined
  • It throws a TypeError

Numbers are stored as binary floating point, so 0.1 + 0.2 is 0.30000000000000004, which is not equal to 0.3.

How it works

  • 20 multiple choice questions, drawn at random from a larger set.
  • 20 minutes on the clock. The test submits itself when time runs out.
  • Pass mark: 60%.
  • No marks are taken off for wrong answers, so answer every question.
  • Some questions have more than one correct answer. You must pick all of them to score.
  • When you finish you see your score, the correct answers and a short explanation for each.
  • Your answers save as you go, so a dropped connection will not lose your work.
  • Please stay on the test page. Switching to another tab or window is recorded and you will see a warning.
  • Copying text, printing and screenshots are not allowed during the test.

More practice tests

See all practice tests.

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