JavaScript Beginner Quiz
Variables, types, functions and arrays — the basics, checked by reading code rather than reciting definitions.
Start the quizSample questions from this quiz
A preview of the style and depth. Try each one, then reveal the answer — or skip straight to the timed quiz.
1. What does this print?
let x = 5
x = x + '5'
console.log(x, typeof x)- 55 number
- 10 number
- 55 string
- NaN number
Show answer
Answer: 55 string. When `+` has a string on either side it concatenates rather than adds, converting the number to text. Every other arithmetic operator does the opposite and converts the string to a number — `5 - '5'` is 0.
2. What does this print?
const arr = [1, 2, 3]
arr.push(4)
console.log(arr.length, arr[0], arr[10])- 4 1 undefined
- 4 1 null
- 3 1 undefined
- 4 0 undefined
Show answer
Answer: 4 1 undefined. `push` adds to the end, so length becomes 4. Arrays are zero-indexed, so `arr[0]` is 1. Reading past the end gives `undefined` rather than throwing — which is why an out-of-range read often shows up much later as a confusing error.
3. This should add two numbers but returns '12'. Which line is wrong?
1 function add(a, b) {
2 return a + b
3 }
4 const total = add(prompt('x'), prompt('y'))- Line 2 — the return needs parentheses
- Line 4 — prompt returns strings, so + concatenates
- Line 1 — parameters need type annotations
- Line 3 — the function is missing a semicolon
Show answer
Answer: Line 4 — prompt returns strings, so + concatenates. `prompt` always yields a string, so `'1' + '2'` gives '12'. Convert first with `Number(...)` or `parseInt(..., 10)`. The same trap applies to values read from form inputs and URL parameters.
Frequently asked questions
How many questions are in this JavaScript quiz?+
30 questions, with a 30-minute time limit. Every question includes a written explanation of the correct answer.
Is this JavaScript quiz free?+
Yes. Every quiz on CodexQuizz is free and needs no account. You only enter a name if you choose to post your score to the leaderboard.
What level is the beginner quiz aimed at?+
Variables, types, functions and arrays — the basics, checked by reading code rather than reciting definitions.
Can I use this to prepare for a JavaScript interview?+
Yes. The questions cover the topics that come up in JavaScript technical screens, and the explanations are written so that a wrong answer still teaches you the underlying concept.
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