Modern JS Talk: Destructuring Assignment

#javascript #es6 #es5

This article was originally published on bmf-tech.com.

※This article is a reprint from the Innovator Japan Engineers’ Blog.

What is Destructuring Assignment

Destructuring assignment is an expression that assigns data from arrays or objects to separate variables. It might be hard to visualize just from the text. Let's look at some examples to understand better.

Array Destructuring

let a, b, c;
[a, b, c] = [1, 2, 3]
console.log(a, b, c) // 1 2 3

let color = [1, 2, 3]
const [red, green, yellow] = color
console.log(red, green, yellow) // 1 2 3

You can intuitively understand it.

You can also set default values for elements extracted from the array that are undefined during destructuring.

const [red=4, green=5, yellow=6] = [1, 2] // when yellow is undefined
console.log(red, green, yellow) // 1, 2, 6

It's like specifying default values for function arguments.

Object Destructuring

({a, b} = {a:'foo', b:'bar'}) // Destructuring assigns 'foo' to variable a and 'bar' to variable b
console.log(a, b) // foo bar

Please refer to the following quote regarding the surrounding (..) in the assignment statement.

The ( .. ) around the assignment statement is necessary syntax when using object literal destructuring without a declaration.

{a, b} = {a:1, b:2} is not valid standalone syntax because the left-hand {a, b} is considered a block, not an object literal.

However, ({a, b} = {a:1, b:2}) is valid because it can be considered as var {a, b} = {a:1, b:2}.

Destructuring Assignment - JavaScript | MDN

Object destructuring is often used like this in React and other frameworks.

let state = {
    value: 'foo'
}
const {value} = state // Destructuring assigns state.value to the variable value
console.log(value) // foo

Intuitively, it looks like this.

const {value} = {value: 'foo'}
console.log(value) // foo

You can also specify default values for object destructuring.

const {foo=3, bar=4} = {foo: 1} // when bar is undefined
console.log(foo, bar) // 1, 4

Furthermore, you can assign values to variables with different names.

const {value: value2} = {value: 'foo'} // Extracts value and assigns it to value2
console.log(value2) // foo

At first glance, const {value} = state might seem confusing, but once you know destructuring, it makes sense! It's convenient and frequently used, so it's worth remembering.