JavaScript ES6+ Features 5

Question 1

What does `async function` always return?

Accepted Answer

A Promise

Answer

An `async` function always wraps its return value in a resolved Promise, and any thrown error becomes a rejected Promise.

Question 2

Which ES2021 method replaces only the first matching substring by default and has a global-replace variant?

Accepted Answer

String.prototype.replaceAll()

Answer

`replaceAll` replaces every occurrence of a substring without needing a global regex flag, complementing `replace` which only replaces the first match with string patterns.

Question 3

What does the optional chaining operator `?.` return when it encounters null or undefined?

Accepted Answer

undefined

Answer

The optional chaining operator `?.` short-circuits and returns `undefined` if the left-hand side is `null` or `undefined`.

Question 4

What is the key difference between ES6 `class` and traditional constructor functions?

Accepted Answer

Class bodies are always in strict mode; constructor functions are not by default

Answer

Code inside a class body executes in strict mode automatically, which is not the case for regular constructor functions in non-strict scripts.

Question 5

What is `Promise.race([p1, p2])` used for?

Accepted Answer

Resolving or rejecting as soon as the first promise settles

Answer

`Promise.race` resolves or rejects with the value/reason of whichever promise settles first, ignoring the rest.

JavaScript Practice Test

JavaScript Practice Test

JavaScript ES6+ Features 5