New Javascript syntax learned

June 23, 20201 min read

1 ) Optional chaining ?.

We have a deeply nested user object below.

let user = {
favorite: {
shoes: {
size: "10.5",
brand: "Nike"
user.favorite?.shoes?.brand // "Nike"
// error
user.favorite.shirts.brand // TypeError: Cannot read property 'brand' of undefined
// no error
user.favorite?.shirts?.brand // undefined

If we want to get user's favourite shirts brand, we would normally write user.favourite.shirt.brand.

But if user has no favourite shirt (shirts property does not exist), getting shirts.brand will return error.

  • If we use optional chaining - user.favorite?.shirts?.brand - we will get undefined instead of error.

  • This is useful for us to call a deeply nested object properties without having an error if any intermediate property in the chain doesn't exist or is undefined.

2 ) Nullish coalescing ??

X ?? Y - Y will be evaluated if X is null or undefined

Prior to this, we use X || Y for short-circuit evaluation.

But the issue for || is that X will be evaluated as false if it is 0 or empty string.

If we need to treat 0 or empty string differently from null or undefined, we should use ?? instead of ||.

let siblings = 0
alert(siblings || "please indicate your no. of siblings")
// "please indicate your no. of siblings"
alert(siblings ?? "please indicate your no. of siblings")
// 0

3 ) Computed property name

It allows us to use expression to dynamically generate property names within object literals.

let fruit = prompt("which is your favorite fruit?") //apple
let basket = {
[fruit]: 5,
[fruit + "juice"]: 10
basket.apple // 5
basket.applejuice // 10

4 ) Property existence test

This is not new, but a neat way to test if a property exists.

let user = {
name: "John",
age: 10
"name" in user // true
"address" in user // false

5 ) Array.find

Find the first element in an array that returns true and returns that element.

const users = [
name: "John",
age: 20
name: "Daisy",
age: 26
users.find((user) => {
return user.name === "Daisy"
// {name: "Daisy", age: 26}

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