Let's talk about the spread operator in ES6

This article brings you relevant knowledge aboutjavascript, which mainly introduces related issues about the expansion operator. The expansion operator of S6 has a very simple syntax. It uses three The dot sign means "...". You can convert an array into a parameter sequence separated by commas. Let's take a look at it. I hope it will be helpful to everyone.

【Related recommendations:javascript video tutorial,web front-end】

ES6’s expansion operator, Its syntax is very simple, using three periods to represent "...". You can convert an array into a comma-separated sequence of parameters.

It expands the iterable object into its individual elements. The so-called iterable object is any object that can be traversed using afor ofloop, such as: array, string,Map,Set,DOMnodes, etc.

Basic syntax

var array = [1,2,3,4]; console.log(...array);//1 2 3 4 var str = "String"; console.log(...str);//S t r i n g

This operator is mainly used for function calls

function push(array, ...items) { array.push(...items); } function add(x, y) { return x + y; } const numbers = [4, 38]; add(...numbers) // 42

In the above code,array.push(...items)andadd(...numbers)These two lines are both function calls, and they both use the spread operator. This operator turns an array into a sequence of parameters.

Expressions can also be placed after the expansion operator

const arr = [ ...(x > 0 ? ['a'] : []), 'b', ];

If the expansion operator is followed by an empty array, it will have no effect.

[...[], 1] // [1]

There are many other uses of the spread operator...

1. The apply method of replacing the array

Using the Math.max() function to obtain the maximum value is:

const m = Math.max(1, 2, 3); //结果为3

Use the apply method combined with Math.max():

But if you want to calculate the maximum value in the array, obviously the array cannot be directly used as a parameter of Math.max(), we need to expand it. Before ES6, we also needed to combine apply to process:

var arr = [2, 4, 8, 6, 0]; function max(arr) { return Math.max.apply(null, arr); } console.log(max(arr));

ES6 can be easily expanded using the spread operator (...). The above example becomes:

var arr = [2, 4, 8, 6, 0]; console.log(Math.max(...arr)); // 3

II , Application of expansion operator

1. Merging arrays

The expansion operator gives us a new method of merging arrays

// ES5 apply 写法 var arr1 = [0, 1, 2]; var arr2 = [3, 4, 5]; Array.prototype.push.apply(arr1, arr2); //arr1 [0, 1, 2, 3, 4, 5]

Using the spread operator can easily expand the array into a parameter list

const a1 = [{ foo: 1 }]; const a2 = [{ bar: 2 }]; const a3 = a1.concat(a2); const a4 = [...a1, ...a2]; a3[0] === a1[0] // true a4[0] === a1[0] // true

In the above code,a3anda4use two different methods The new arrays are merged, but their members are references to the original array members. This is a shallow copy. If the value pointed to by the reference is modified, it will be reflected in the new array synchronously.

Note: These two methods are shallow copies, so you need to pay attention when using them.

2. Copy Array

Array is a composite data type. If you copy it directly, you only copy the pointer to the underlying data structure. Not cloning a completely new array.

ES5 can only use workarounds to copy arrays.

const a1 = [1, 2]; const a2 = a1.concat(); a2[0] = 2; a1 // [1, 2]

In the above code,a1will return a clone of the original array, and modifyinga2will not affecta1.

The spread operator provides a simple way to copy an array.

//拷贝数组 var array0 = [1,2,3]; var array1 = [...array0]; console.log(array1);//[1, 2, 3] //拷贝数组 var obj = { age:1, name:"lis", arr:{ a1:[1,2] } } var obj2 = {...obj}; console.log(obj2);//{age: 1, name: "lis", arr: {…}}

Remember: the array is still obtained through the pointer, so we do not copy the array itself, we copy just a new pointer.

3. Convert pseudo array to array

NodeListThe object is a collection of nodes, usually Returned by properties such asNode.childNodesand methods such asdocument.querySelectorAll.

像 NodeList 和 arguments 这种伪数组，类似于数组，但不是数组，没有Array的所有方法，例如find、map、filter等，但是可以使用forEach()来迭代

可以通过扩展运算符将其转为数组，如下：

const nodeList = document.querySelectorAll(".row"); const nodeArray = [...nodeList]; console.log(nodeList); console.log(nodeArray);

注意：使用扩展运算符将伪数组转换为数组有局限性，这个类数组必须得有默认的迭代器且伪可遍历的

4.与解构赋值结合

扩展运算符可以与解构赋值结合起来，用于生成数组

// ES5 a = list[0], rest = list.slice(1) // ES6 [a, ...rest] = list

下面是另外一些例子：

const [first, ...rest] = [1, 2, 3, 4, 5]; first // 1 rest // [2, 3, 4, 5] const [first, ...rest] = []; first // undefined rest // [] const [first, ...rest] = ["foo"]; first // "foo" rest // []

注意：如果将扩展运算符用于数组赋值，只能放在参数的最后一位，否则会报错。

const [...butLast, last] = [1, 2, 3, 4, 5]; // 报错 const [first, ...middle, last] = [1, 2, 3, 4, 5]; // 报错

5. 字符串

ES6的扩展语法可以很简单的把一个字符串分割为单独字符的数组：

[...'hello'] // [ "h", "e", "l", "l", "o" ]

6.Map 和 Set 结构，Generator 函数

扩展运算符内部调用的是数据结构的 Iterator 接口，因此只要具有 Iterator 接口的对象，都可以使用扩展运算符，比如 Map 结构。

let map = new Map([ [1, 'one'], [2, 'two'], [3, 'three'], ]); let arr = [...map.keys()]; // [1, 2, 3]

Generator 函数运行后，返回一个遍历器对象，因此也可以使用扩展运算符。

var go = function*(){ yield 1; yield 2; yield 3; }; [...go()] // [1, 2, 3]

上面代码中，变量go是一个 Generator 函数，执行后返回的是一个遍历器对象，对这个遍历器对象执行扩展运算符，就会将内部遍历得到的值，转为一个数组。

如果对没有 Iterator 接口的对象，使用扩展运算符，将会报错。

const obj = {a: 1, b: 2}; let arr = [...obj]; // TypeError: Cannot spread non-iterable object

【相关推荐：javascript视频教程、web前端】

The above is the detailed content of Let's talk about the spread operator in ES6. For more information, please follow other related articles on the PHP Chinese website!