Multiple choice practice

ES6+ MCQ questions

Take a short practice test with instant feedback, then revise every question with its answer and the reason behind it.

ES6+ tutorial Interview questions

Questions: 84
From lessons: 64
Your last test: —

Practice test

ES6+ practice testTrue or false Question 1 of 10

A rest parameter is a real array with map, filter and reduce.

Score so far: 0/0

All questions with answers

Which ECMAScript edition is commonly called ES6?
- AES2009
- BES2015
- CES2016
- DES2020
Show answer
Answer: B. ES2015
ES6 is ECMAScript 2015; editions have been yearly since.
What is logged?
JavaScript Illustrative
```
1const n = 1;
2n = 2;
3console.log(n);
```
- A1
- B2
- CTypeError
- Dundefined
Show answer
Answer: C. TypeError
Reassigning a const binding throws “Assignment to constant variable”.
What does this arrow function return?
JavaScript Illustrative
```
1const make = () => { id: 1 };
```
- AAn object { id: 1 }
- Bundefined
- C1
- DA syntax error
Show answer
Answer: B. undefined
Braces start a function body with a label “id”. Wrap the object in parentheses: () => ({ id: 1 }).
What does `Total: ${2 + 3}` produce?
- A"Total: 2 + 3"
- B"Total: 5"
- C"Total: ${5}"
- DAn error
Show answer
Answer: B. "Total: 5"
Expressions inside ${} are evaluated and converted to strings.
What does const [a, ...rest] = [1, 2, 3] give rest?
- A2
- B[2, 3]
- C[1, 2, 3]
- D3
Show answer
Answer: B. [2, 3]
The rest element collects the remaining items into an array.
Which syntax merges two objects into a new one?
- AObject.merge(a, b)
- B{ ...a, ...b }
- Ca + b
- D[...a, ...b]
Show answer
Answer: B. { ...a, ...b }
Object spread copies own enumerable properties; later ones win.
What does Math.max(...[4, 9, 2]) return?
- A[4, 9, 2]
- B9
- CNaN
- D4
Show answer
Answer: B. 9
Spread passes array items as separate arguments.
Which keyword calls the parent constructor in a class?
- Aparent()
- Bbase()
- Csuper()
- Dthis.parent()
Show answer
Answer: C. super()
super() must be called before using this in a derived constructor.
What does user?.address?.city return if user is null?
- Anull
- Bundefined
- CTypeError
- D""
Show answer
Answer: B. undefined
Optional chaining short-circuits to undefined.
What does 0 ?? 5 evaluate to?
- A5
- B0
- Cnull
- Dundefined
Show answer
Answer: B. 0
?? only falls back for null or undefined; 0 is kept.
What does new Set([1, 1, 2]).size return?
- A3
- B2
- C1
- Dundefined
Show answer
Answer: B. 2
A Set stores unique values only.
Which keys can a Map use?
- AStrings only
- BStrings and symbols
- CAny value, including objects
- DNumbers only
Show answer
Answer: C. Any value, including objects
Map keys can be any type and are compared with SameValueZero.
What does a generator function return when called?
- AIts first yielded value
- BAn iterator object
- CA promise
- Dundefined
Show answer
Answer: B. An iterator object
Calling it does not run the body; it returns a generator you drive with next().
What does Symbol("id") === Symbol("id") evaluate to?
- Atrue
- Bfalse
- CTypeError
- Dundefined
Show answer
Answer: B. false
Every Symbol() call creates a unique value; use Symbol.for for shared ones.
Which export can be imported with any name and without braces?
- AA named export
- BA default export
- CA namespace export
- DA re-export
Show answer
Answer: B. A default export
import anything from "./file.js" receives the default export.
What does import("./chart.js") return?
- AThe module object directly
- BA promise of the module
- Cundefined
- DA string path
Show answer
Answer: B. A promise of the module
Dynamic import is asynchronous and returns a promise.
Which method returns the last element of an array?
- Aarr.last()
- Barr.at(-1)
- Carr[-1]
- Darr.end()
Show answer
Answer: B. arr.at(-1)
at() accepts negative indexes; arr[-1] reads a property named "-1".
Which method sorts without changing the original array?
- Asort()
- BtoSorted()
- Corder()
- Dsorted()
Show answer
Answer: B. toSorted()
toSorted (ES2023) returns a new sorted array.
What is logged?
JavaScript Illustrative
```
1function greet(name = 'guest') { return name; }
2console.log(greet(null));
```
- Aundefined
- B"guest"
- Cnull
- D""
Show answer
Answer: C. null
Default parameters only apply for undefined, so null is passed through.
What does Object.entries({ a: 1 }) return?
- A["a", 1]
- B[["a", 1]]
- C{ a: 1 }
- D["a"]
Show answer
Answer: B. [["a", 1]]
An array of [key, value] pairs, which pairs well with for…of and Object.fromEntries.
What is logged?
JavaScript Illustrative
```
1const { a = 1, b = 2 } = { a: null };
2console.log(a, b);
```
- A1 2
- Bnull 2
- Cundefined 2
- Dnull null
Show answer
Answer: B. null 2
Defaults only replace undefined. a is null, so it stays null; b is missing, so it defaults to 2.
From the lesson: Destructuring
What is logged?
JavaScript Illustrative
```
1console.log(0 ?? 5, 0 || 5);
```
- A0 0
- B5 5
- C0 5
- D5 0
Show answer
Answer: C. 0 5
?? only replaces null/undefined, so 0 stays. || replaces any falsy value.
From the lesson: Optional Chaining
ES6 is another name for…
- AES2009
- BES2015
- CES2020
- DTypeScript
Show answer
Answer: B. ES2015
ES6 was officially renamed ECMAScript 2015.
From the lesson: Introduction to ECMAScript
Which committee decides new JavaScript features?
- AW3C
- BWHATWG
- CTC39
- DNode.js Foundation
Show answer
Answer: C. TC39
TC39 (Ecma Technical Committee 39) maintains ECMAScript.
From the lesson: Introduction to ECMAScript
async/await was introduced in ES2015.
- ATrue
- BFalse
Show answer
Answer: B. False
async/await arrived in ES2017. ES2015 introduced promises.
From the lesson: Introduction to ECMAScript