Array.from()
方法从一个类似数组或可迭代对象创建一个新的,浅拷贝的数组实例。
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语法
Array.from(arrayLike[, mapFn[, thisArg]])
参数
-
arrayLike
- 想要转换成数组的伪数组对象或可迭代对象。
-
mapFn
可选 - 如果指定了该参数,新数组中的每个元素会执行该回调函数。
-
thisArg
可选 -
可选参数,执行回调函数
mapFn
时this
对象。
返回值
一个新的数组
实例。
描述
Array.from()
可以通过以下方式来创建数组对象:
- 伪数组对象(拥有一个
length
属性和若干索引属性的任意对象) - 可迭代对象(可以获取对象中的元素,如 Map和 Set 等)
Array.from()
方法有一个可选参数 mapFn
,让你可以在最后生成的数组上再执行一次 map
方法后再返回。也就是说 Array.from(obj, mapFn, thisArg)
就相当于 Array.from(obj).map(mapFn, thisArg),
除非创建的不是可用的中间数组。 这对一些数组的子类,
如 typed arrays 来说很重要, 因为中间数组的值在调用 map() 时需要是适当的类型。
from()
的 length
属性为 1 ,即 Array.from.length === 1
。
在 ES2015 中, Class
语法允许我们为内置类型(比如 Array
)和自定义类新建子类(比如叫 SubArray
)。这些子类也会继承父类的静态方法,比如 SubArray.from()
,调用该方法后会返回子类 SubArray
的一个实例,而不是 Array
的实例。
示例
从 String
生成数组
Array.from('foo'); // [ "f", "o", "o" ]
从 Set
生成数组
const set = new Set(['foo', 'bar', 'baz', 'foo']); Array.from(set); // [ "foo", "bar", "baz" ]
从 Map
生成数组
const map = new Map([[1, 2], [2, 4], [4, 8]]); Array.from(map); // [[1, 2], [2, 4], [4, 8]] const mapper = new Map([['1', 'a'], ['2', 'b']]); Array.from(mapper.values()); // ['a', 'b']; Array.from(mapper.keys()); // ['1', '2'];
从类数组对象(arguments)生成数组
function f() { return Array.from(arguments); } f(1, 2, 3); // [ 1, 2, 3 ]
在 Array.from
中使用箭头函数
// Using an arrow function as the map function to // manipulate the elements Array.from([1, 2, 3], x => x + x); // [2, 4, 6] // Generate a sequence of numbers // Since the array is initialized with `undefined` on each position, // the value of `v` below will be `undefined` Array.from({length: 5}, (v, i) => i); // [0, 1, 2, 3, 4]
Sequence generator (range)
// Sequence generator function (commonly referred to as "range", e.g. Clojure, PHP etc) const range = (start, stop, step) => Array.from({ length: (stop - start) / step + 1}, (_, i) => start + (i * step)); // Generate numbers range 0..4 range(0, 4, 1); // [0, 1, 2, 3, 4] // Generate numbers range 1..10 with step of 2 range(1, 10, 2); // [1, 3, 5, 7, 9] // Generate the alphabet using Array.from making use of it being ordered as a sequence range('A'.charCodeAt(0), 'Z'.charCodeAt(0), 1).map(x => String.fromCharCode(x)); // ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"]
数组去重合并
function combine(){ let arr = [].concat.apply([], arguments); //没有去重复的新数组 return Array.from(new Set(arr)); } var m = [1, 2, 2], n = [2,3,3]; console.log(combine(m,n)); // [1, 2, 3]
Polyfill
ECMA-262 第六版标准中添加了 Array.from
。有些实现中可能尚未包括在其中。你可以通过在脚本前添加如下内容作为替代方法,以使用未原生支持的 Array.from
方法。该算法按照 ECMA-262 第六版中的规范实现,并假定 Object
和 TypeError
有其本身的值, callback.call
对应 Function.prototype.call
。此外,鉴于无法使用 Polyfill 实现真正的的迭代器,该实现不支持规范中定义的泛型可迭代元素。
// Production steps of ECMA-262, Edition 6, 22.1.2.1 if (!Array.from) { Array.from = (function () { var toStr = Object.prototype.toString; var isCallable = function (fn) { return typeof fn === 'function' || toStr.call(fn) === '[object Function]'; }; var toInteger = function (value) { var number = Number(value); if (isNaN(number)) { return 0; } if (number === 0 || !isFinite(number)) { return number; } return (number > 0 ? 1 : -1) * Math.floor(Math.abs(number)); }; var maxSafeInteger = Math.pow(2, 53) - 1; var toLength = function (value) { var len = toInteger(value); return Math.min(Math.max(len, 0), maxSafeInteger); }; // The length property of the from method is 1. return function from(arrayLike/*, mapFn, thisArg */) { // 1. Let C be the this value. var C = this; // 2. Let items be ToObject(arrayLike). var items = Object(arrayLike); // 3. ReturnIfAbrupt(items). if (arrayLike == null) { throw new TypeError("Array.from requires an array-like object - not null or undefined"); } // 4. If mapfn is undefined, then let mapping be false. var mapFn = arguments.length > 1 ? arguments[1] : void undefined; var T; if (typeof mapFn !== 'undefined') { // 5. else // 5. a If IsCallable(mapfn) is false, throw a TypeError exception. if (!isCallable(mapFn)) { throw new TypeError('Array.from: when provided, the second argument must be a function'); } // 5. b. If thisArg was supplied, let T be thisArg; else let T be undefined. if (arguments.length > 2) { T = arguments[2]; } } // 10. Let lenValue be Get(items, "length"). // 11. Let len be ToLength(lenValue). var len = toLength(items.length); // 13. If IsConstructor(C) is true, then // 13. a. Let A be the result of calling the [[Construct]] internal method // of C with an argument list containing the single item len. // 14. a. Else, Let A be ArrayCreate(len). var A = isCallable(C) ? Object(new C(len)) : new Array(len); // 16. Let k be 0. var k = 0; // 17. Repeat, while k < len… (also steps a - h) var kValue; while (k < len) { kValue = items[k]; if (mapFn) { A[k] = typeof T === 'undefined' ? mapFn(kValue, k) : mapFn.call(T, kValue, k); } else { A[k] = kValue; } k += 1; } // 18. Let putStatus be Put(A, "length", len, true). A.length = len; // 20. Return A. return A; }; }()); }
规范
Specification | Status | Comment |
---|---|---|
ECMAScript Latest Draft (ECMA-262) Array.from |
Draft | |
ECMAScript 2015 (6th Edition, ECMA-262) Array.from |
Standard | Initial definition. |
浏览器兼容性
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
from |
Chrome Full support 45 | Edge Full support 12 | Firefox Full support 32 | IE No support No | Opera Full support Yes | Safari Full support 9 | WebView Android Full support 45 | Chrome Android Full support 45 | Firefox Android Full support 32 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support Yes | nodejs Full support 4.0.0 |
Legend
- Full support
- Full support
- No support
- No support