Small knowledge, big challenge! This article is participating in the creation activity of “Essential Tips for Programmers”.

  1. Deconstruction:

That is to extract a variable from an object or array and assign it to it

  1. Why:

Extracting data objects requires assignment by assignment, which may mine the entire structure for a small piece of data. ES6 adds deconstruction to arrays and objects to facilitate extracting data

3.# Object and function argument deconstruction

An array of deconstruction

  1. low
let a = 1
let b = 2
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  1. general
let a = 1, b = 2
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  1. good
let [a, b] = [1, 2]
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Similar to numeric swapping, it can be implemented in one line of code

let [a, b] = [1, 2]
[a, b] = [b, a]
console.log(a, b) // 2, 1
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If (let [a] = 1, let [a] = 1) TypeError: This is a normal case, and sometimes some special cases are encountered

let [ , , a] = [0, 1, 2]
console.log(a) // 2
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A corresponds to the 2 on the right, so it’s also going to be 2

let [a, ,b] = [1, 2, 3]
console.log(a) // 1
console.log(b) // 3
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The middle is undefined, of course, so a is at 1 and B is at 3

let [a, b, c] = [1]
console.log(a) // 1
console.log(b) // undefined
console.log(c) // undefined
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If there is no value, the value will be undefined by default, so a will be 1, b and C will be undefined

let [a, b, ...c] = [1, null]
console.log(a) // 1
console.log(b) // null
console.log(c) // []
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C is an array, if empty, an empty array, b is a non-array, if empty, undefined, and if null, null

let [a, ...b] = [1, 2, 3, 4]
console.log(a) // 1
console.log(b) // [2, 3, 4]
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An array can eat all the values behind it but it can’t be defined at the end of the array, so you get an error, for example

let [...a, b] = [1, 2, 3, 4] 
// error: Rest element must be last element
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let [a = 1, b] = []
console.log(a) // 1
console.log(b) // undefined
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When there is a default value in the structure, the default value takes precedence, but there is one case where null is overridden

let [a = 1] = [null]
console.log(a) // null
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