JavaScript Quiz – Test Your Coding Knowledge

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JavaScript

JavaScript Quiz – 20 Questions to Test Your Skills

Test your JavaScript knowledge with this 20-question quiz. Perfect for beginners and experienced developers to practice, learn, and sharpen their JS skills.

1 / 20

What is logged?

const target = { a: 1 };
const handler = {
  get: function(obj, prop) {
    return prop in obj ? obj[prop] : 42;
  }
};
const p = new Proxy(target, handler);
console.log(p.a, p.b);

1. 1 undefined

2. undefined 42

3. Throws an error

4. 1 42

2 / 20

How does prototypal inheritance work in JavaScript?

1. Objects inherit properties and methods from a parent class blueprint defined with the class keyword.

2. Objects can inherit properties and methods directly from other objects through an internal [[Prototype]] link.

3. Inheritance is achieved by copying all properties and methods from a parent object to a child object at creation.

4. JavaScript does not support inheritance; it only supports object composition.

3 / 20

What is the primary role of the Event Loop in JavaScript's runtime environment?

1. To compile JavaScript code into machine code for faster execution.

2. To handle all mathematical computations and offload them to the computer's GPU.

3. To manage the call stack, callback queue, and microtask queue, ensuring non-blocking asynchronous operations.

4. To prevent memory leaks by periodically garbage collecting unused variables.

4 / 20

In JavaScript, what is the primary purpose of "truthy" and "falsy" value evaluation?

1. To enforce strict type checking in all conditional statements.

2. To allow non-Boolean values to be used in contexts that require a Boolean, enabling more flexible and concise code.

3. To automatically convert all values to strings for comparison.

4. It is a legacy feature with no practical purpose in modern JavaScript.

5 / 20

What does this code most likely output?

const formatter = new Intl.NumberFormat('de-DE', { style: 'currency', currency: 'EUR' });
console.log(formatter.format(123456.789));

1. "€123,456.79"

2. "123.456,79 €"

3. "123 456,79 €"

4. "123.456,79€"

6 / 20

Why would you use a WeakMap over a Map?

1. For better iteration performance.

2. To allow keys of any data type.

3. To prevent memory leaks by allowing keys to be garbage collected.

4. To have built-in sorting of keys.

7 / 20

What is the result?

const arrLike = { length: 3 };
const result = Array.from(arrLike, (v, i) => i * 2);
console.log(result);

1. [0, 2, 4]

2. [undefined, undefined, undefined]

3. [null, null, null]

4. []

8 / 20

let and the Temporal Dead Zone (TDZ)

console.log(example);
let example = "value";

1. undefined

2. "value"

3. Throws a ReferenceError

4. Throws a SyntaxError

9 / 20

What does this code output?

const result = ['1', '2', '3'].map(parseInt); console.log(result);

1. [1, 2, 3]

2. ['1', '2', '3']

3. [1, NaN, NaN]

4. Throws an error

10 / 20

In what order is the output logged?

1. 1, 2, 3, 4

2. 1, 4, 3, 2

3. 1, 4, 2, 3

4. 1, 4, 3, 2

11 / 20

What is the output?

function* generator() { yield 1; yield* [2, 3]; yield 4; } console.log([...generator()].join('-'));

1. "1-2-3-4"

2. "1-[2,3]-4"

3. "1-2,3-4"

4. Throws an error

12 / 20

What is the purpose of the use strict directive?

1. Makes JavaScript run faster

2. Enables stricter parsing and error handling

3. Converts JavaScript to TypeScript

4. Allows using newer ES6 features

13 / 20

What does console.log(2 + "2" - 1) output?

1. "21"

2. 21

3. "221"

4. 3

14 / 20

Which symbol is used for strict equality comparison?

1. =

2. ==

3. ===

4. !==

15 / 20

What is the output of console.log([] == ![])?

1. true

2. false

3. undefined

4. Error

16 / 20

Which method creates a new array with all elements that pass a test?

1. map()

2. filter()

3. reduce()

4. forEach()

17 / 20

What will console.log(0.1 + 0.2 === 0.3) output?

1. true

2. false

3. undefined

4. Error

18 / 20

What does "5" + 3 return in JavaScript?

1. 8

2. 53

3. "53"

4. Error

19 / 20

Which method adds one or more elements to the end of an array?

1. push()

2. pop()

3. shift()

4. append()

20 / 20

What will console.log(typeof null) output?

1. "object"

2. "null"

3. "undefined"

4. "string"

Your score is

The average score is 15%