Spread operator

扩展语法允许一个表达式在期望多个参数（用于函数调用）或多个元素（用于数组字面量）或多个变量（用于解构赋值）的位置扩展。

语法

用于函数调用:

myFunction(...iterableObj);

用于数组字面量:

[...iterableObj, 4, 5, 6];

用于对象字面量 (new in ECMAScript; stage 3 draft):

let objClone = { ...obj };

范例

在函数调用中

更好的 apply 方法

在需要使用数组作为函数的参数的情况下,通常使用 Function.prototype.apply 方法:

function myFunction(x, y, z) { }
var args = [0, 1, 2];
myFunction.apply(null, args);

如果使用了ES6的展开运算符,你可以这么写:

function myFunction(x, y, z) { }
var args = [0, 1, 2];
myFunction(...args);

就像扩展参数列表一样，...可以在数组字面量中的任何地方使用，可以多次使用。

function myFunction(v, w, x, y, z) { }
var args = [0, 1];
myFunction(-1, ...args, 2, ...[3]);

配合new运算符

例子:在ES5中,我们无法同时使用 new 运算符和 apply 方法(apply方法调用[[Call]]而不是[[Construct]])。在ES6中，我们可以使用扩展运算符，和普通的函数调用一样。

var dateFields = [1970, 0, 1];  // 1 Jan 1970
var d = new Date(...dateFields);

要使用新的参数数组而不使用扩展语法，则必须通过部分应用程序间接执行它：

function applyAndNew(constructor, args) {
   function partial () {
      return constructor.apply(this, args);
   };
   if (typeof constructor.prototype === "object") {
      partial.prototype = Object.create(constructor.prototype);
   }
   return partial;
}


function myConstructor () {
   console.log("arguments.length: " + arguments.length);
   console.log(arguments);
   this.prop1="val1";
   this.prop2="val2";
};

var myArguments = ["hi", "how", "are", "you", "mr", null];
var myConstructorWithArguments = applyAndNew(myConstructor, myArguments);

console.log(new myConstructorWithArguments);
// (internal log of myConstructor):           arguments.length: 6
// (internal log of myConstructor):           ["hi", "how", "are", "you", "mr", null]
// (log of "new myConstructorWithArguments"): {prop1: "val1", prop2: "val2"}

Spread in array literals

一个更好的连接数组的方法

无spread的语法，使用现有阵列作为它的一个部分创建一个新的数组，数组文本语法不再足够和命令性代码必须使用的组合来代替push，splice，concat等。随着扩展语法此变得更加简洁：

var parts = ['shoulders', 'knees']; 
var lyrics = ['head', ...parts, 'and', 'toes']; 
// ["head", "shoulders", "knees", "and", "toes"]

就像传递参数列表一样，...可以在数组文字中的任何位置使用，并且可以多次使用。

复制一个数组

var arr = [1, 2, 3];
var arr2 = [...arr]; // like arr.slice()
arr2.push(4); 

// arr2 becomes [1, 2, 3, 4]
// arr remains unaffected

Note:复制数组时候, 拓展语句只会进行浅复制, 因此如下所示, 它并不适合复制多维数组 (与Object.assign()相同)。

var a = [[1], [2], [3]];
var b = [...a];
b.shift().shift(); // 1
// Now array a is affected as well: [[], [2], [3]]

A better way to concatenate arrays

Array.concat通常用于将数组连接到现有数组的末尾。如果没有扩展语法，这是完成的：

var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
// Append all items from arr2 onto arr1
arr1 = arr1.concat(arr2);

有spread语法：

var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
arr1 = [...arr1, ...arr2];

Array.unshift通常用于在现有数组的开始插入一个数组值。如果没有扩展语法，这是完成的：

var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
// Prepend all items from arr2 onto arr1
Array.prototype.unshift.apply(arr1, arr2) // arr1 is now [3, 4, 5, 0, 1, 2]

With spread syntax this becomes:

var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
arr1 = [...arr2, ...arr1]; // arr1 is now [3, 4, 5, 0, 1, 2]

Spread in object literals

The Rest/Spread Properties for ECMAScript proposal (stage 3) adds spread properties to object literals. It copies own enumerable properties from a provided object onto a new object.

Shallow-cloning (excluding prototype) or merging of objects is now possible using a shorter syntax than Object.assign().

var obj1 = { foo: 'bar', x: 42 };
var obj2 = { foo: 'baz', y: 13 };

var clonedObj = { ...obj1 };
// Object { foo: "bar", x: 42 }

var mergedObj = { ...obj1, ...obj2 };
// Object { foo: "baz", x: 42, y: 13 }

Note that Object.assign() triggers setters whereas spread syntax doesn't.

Only for iterables

Spread syntax (other than in the case of spread properties) can be applied only to iterable objects:

var obj = {'key1': 'value1'};
var array = [...obj]; // TypeError: obj is not iterable

Spread with many values

When using spread syntax for function calls, be aware of the possibility of exceeding the JavaScript engine's argument length limit. See apply() for more details.

剩余操作符

还有一种操作符叫做剩余操作符（the rest operator），它的样子看起来和展开操作符一样，但是它是用于解构数组和对象。在某种程度上，剩余元素和展开元素相反，展开元素会“展开”数组变成多个元素，剩余元素会收集多个元素和“压缩”成一个单一的元素。

规范

Specification	Status	Comment
ECMAScript 2015 (6th Edition, ECMA-262)	Standard	Defined in several sections of the specification: Array Initializer, Argument Lists
ECMAScript Latest Draft (ECMA-262)	Living Standard	No changes.
Rest/Spread Properties for ECMAScript	Draft	Stage 3 draft.

浏览器兼容性

Feature	Chrome	Edge	Firefox (Gecko)	Internet Explorer	Opera	Safari (WebKit)
Spread in array literals	46	20 (12.10240)	16 (16)	No support	37	7.1
Spread in function calls	46	20 (12.10240)	27 (27)	No support	37	7.1
Spread in destructuring	49	No support	34 (34)	No support	37	?
Spread in object literals	60	No support	55 (55)	No support	No support	No support

Feature	Android Webview	Edge	Firefox Mobile (Gecko)	IE Mobile	Opera Mobile	Safari Mobile	Chrome for Android
Spread in array literals	46	20 (12.10240)	16.0 (16)	No support	No support	8	46
Spread in function calls	46	20 (12.10240)	27.0 (27)	No support	No support	8	46
Spread in destructuring	No support	No support	34.0 (34)	?	?	?	No support
Spread in object literals	No support	No support	55.0 (55)	No support	No support	No support	No support

本文档系腾讯云开发者社区成员共同维护，如有问题请联系 cloudcommunity@tencent.com

最后更新于：2017-12-18