javascriptfrom()方法将一个类数组对象转换成真实的数组

Array.from(arrayLike[, mapFn[, thisArg]])

// 将类数组对象（arguments）转换成数组
(function () {
 var args = Array.from(arguments);
 return args;
})(1, 2, 3); // [1, 2, 3]

// 将可迭代对象（Set 对象）转换成数组
Array.from(Set(["foo", window])); // ["foo", window]

// Map 对象也可以
var m = new Map([[1, 2], [2, 4], [4, 8]]);
Array.from(m); // [[1, 2], [2, 4], [4, 8]] 

// 字符串对象既是类数组又是可迭代对象
Array.from("foo"); // ["f", "o", "o"]

// 使用 map 函数转换数组元素
Array.from([1, 2, 3], x => x + x); // [2, 4, 6]

// 生成一个数字序列
Array.from({length:5}, (v, k) => k); // [0, 1, 2, 3, 4]

// Production steps of ECMA-262, Edition 6, 22.1.2.1
// Reference: https://people.mozilla.org/~jorendorff/es6-draft.html#sec-array.from
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;
 };
 }());
}