Array.from()

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。

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 mapFnthis 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,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()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

String 生成数组

Array.from('foo'); 
// [ "f", "o", "o" ]

Set 生成数组

const set = new Set(['foo', 'bar', 'baz', 'foo']);
Array.from(set);
// [ "foo", "bar", "baz" ]

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 ]

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 ObjectTypeError 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.

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fromChrome Full support 45Edge Full support 12Firefox Full support 32IE No support NoOpera Full support YesSafari Full support 9WebView Android Full support 45Chrome Android Full support YesFirefox Android Full support 32Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support 4.0.0

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