ES6+ Quiz - Languages JavaScript

Free practice quiz: ES6+ (Languages JavaScript). 56 multiple-choice questions with answers and explanations. Play online, no sign-up needed.

This Languages JavaScript practice quiz has 56 multiple-choice questions on ES6+, and 56 of them come with an explanation. Try the 5 sample questions below (tap an option to check yourself), then play the full quiz online. No sign-up needed.

Sample questions

1. Which of the following correctly declares a constant variable in JavaScript ES6?

   1. var PI = 3.14;
   2. let PI = 3.14;
   3. const PI = 3.14;
   4. constant PI = 3.14;
   Show answer

   Answer: C. const PI = 3.14;

   In ES6, the `const` keyword is used to declare variables whose values cannot be reassigned. `var` and `let` declare variables that can be reassigned, and `constant` is not a valid JavaScript keyword for variable declaration.

2. What does the `async` keyword in ES6 signify?

   1. It declares a function that will always return a string
   2. It declares a function that will automatically pause execution until a Promise resolves
   3. It declares a function that implicitly returns a Promise
   4. It declares a function that is asynchronous by nature
   Show answer

   Answer: C. It declares a function that implicitly returns a Promise

   An `async` function always returns a Promise. If the function returns a non-Promise value, it's wrapped in a resolved Promise. If it throws an error, it returns a rejected Promise.

3. Consider the code: `let x = 10; function outer() { let x = 20; function inner() { console.log(x); } inner(); } outer();`. What is logged?

   1. 10
   2. 20
   3. undefined
   4. Error
   Show answer

   Answer: B. 20

   JavaScript uses lexical scoping. The `inner` function is within the scope of `outer`, which has its own `x` variable with the value `20`. Therefore, `inner` accesses `x` from `outer`'s scope, not the global scope.

4. What is the purpose of the `Symbol.iterator` method in ES6?

   1. To define how an object can be iterated over
   2. To create a new symbol
   3. To perform mathematical operations
   4. To handle asynchronous operations
   Show answer

   Answer: A. To define how an object can be iterated over

   The `Symbol.iterator` is a method that is called when a `for...of` loop or other iterable consumption methods are used on an object. It returns an iterator object, which implements the iterator protocol.

5. What is a WeakMap in ES6?

   1. A Map where all keys must be weak references
   2. A Map where keys are weakly held, allowing garbage collection if the key is no longer referenced elsewhere
   3. A Map that only stores weak references
   4. A Map that throws an error if a key is too large
   Show answer

   Answer: B. A Map where keys are weakly held, allowing garbage collection if the key is no longer referenced elsewhere

   Unlike a `Map`, keys in a `WeakMap` must be objects and are held weakly. If an object used as a key is garbage collected, the entry in the `WeakMap` is automatically removed.

More questions in this quiz

Answers and explanations are in the full quiz.

1. What is the output of `console.log([...[1, 2], ...[3, 4]]);` in ES6?
2. What does the arrow function `((a, b) => a + b)(5, 3)` return?
3. Which statement correctly uses template literals in ES6?
4. What is the purpose of the `let` keyword in ES6?
5. Consider the following code: `const arr = [1, 2, 3]; const [a, b] = arr; console.log(a);`. What will be logged?
6. What is a default parameter in JavaScript ES6?
7. What does `const { name, age } = { name: 'Alice', age: 30 };` do?
8. Which of the following is a valid ES6 class declaration?
9. What is the output of `console.log(Promise.resolve(5))`?
10. What is a generator function in ES6?
11. What does the `await` keyword in ES6 do?
12. What is a Symbol in ES6?
13. What does the `Map` object in ES6 represent?
14. What is the purpose of the `Set` object in ES6?
15. What is the output of `console.log(String.fromCharCode(65))`?
16. Which ES6 feature allows you to define a function that takes a variable number of arguments as an array?
17. What is `Object.assign()` used for in ES6?
18. What is the output of `console.log(Math.max(...[1, 5, 2]))`?
19. Which of the following is NOT a new primitive type introduced in ES6?
20. What is the difference between `Array.prototype.map()` and `Array.prototype.forEach()` in ES6?
21. What does the `finally` block in a `try...catch...finally` statement do?
22. What is the output of `console.log(typeof null)`?
23. What is the primary benefit of using `const` over `let`?
24. Which of these is NOT a valid way to create an object in JavaScript?
25. What is the result of `console.log(2 + '3' * 2)`?
26. Which ES6 feature is used to create promises that can be chained together?
27. What is the primary difference between `let` and `var` regarding scope in ES6?
28. What will `console.log('hello'.repeat(3))` output?
29. Which ES6 method can be used to check if an array includes a certain element?
30. What is the output of `console.log(0.1 + 0.2 === 0.3)`?
31. What is the output of `const x = 10; if (true) { let x = 20; } console.log(x);`?
32. Which of the following is an example of a default parameter in an ES6 arrow function?
33. What is the difference between `Object.keys()` and `Object.values()` in ES6?
34. What does the `entries()` method of a Map return in ES6?
35. What is the result of `console.log('abc'.padStart(5, 'x'))`?
36. Which ES6 feature is used to create a shallow copy of an array?
37. What does the `for...of` loop iterate over in ES6?
38. What is the purpose of `Object.entries()` in ES6?
39. What is the output of `console.log(String.prototype.includes.call('JavaScript', 'Script'))`?
40. What is a Proxy in ES6?
41. What does the `Array.from()` method do in ES6?
42. What is the primary purpose of the `Reflect` object in ES6?
43. What does `const myString = 'Hello'; console.log(myString[0]);` output?
44. Which ES6 feature provides a more direct way to handle promises than traditional callback chaining?
45. What is the output of `console.log(typeof typeof 1);`?
46. What does the `Object.getOwnPropertyDescriptor()` method return for a given property of an object?
47. What is the result of `let a = [1, 2]; let b = a; b.push(3); console.log(a);`?
48. Which ES6 feature allows for cleaner syntax for creating functions that return values when immediately invoked?
49. What is the output of `console.log(typeof NaN)`?
50. What does the `Array.prototype.fill()` method do in ES6?
51. Which ES6 feature allows you to define a function that returns a Promise and can pause its execution until another Promise resolves?

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