array.from js

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

// 类数组对象转数组
function demoFunction() {
    var arr = Array.from(arguments);
    console.log(arr); // 输出传入的参数组成的数组
}

demoFunction(1, 2, 3); // [1, 2, 3]

// 字符串转数组
var str = 'hello';
var arrFromStr = Array.from(str);
console.log(arrFromStr); // ['h', 'e', 'l', 'l', 'o']

// 生成数值范围数组
var range = (start, end) => Array.from({ length: end - start + 1 }, (_, i) => i + start);
console.log(range(1, 5)); // [1, 2, 3, 4, 5]

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);
    };

    return function from(arrayLike/*, mapFn, thisArg */) {
      var C = this;
      var items = Object(arrayLike);
      if (arrayLike == null) {
        throw new TypeError('Array.from requires an array-like object - not null or undefined');
      }
      var mapFn = arguments.length > 1 ? arguments[1] : void undefined;
      var T;
      if (typeof mapFn !== 'undefined') {
        if (!isCallable(mapFn)) {
          throw new TypeError('Array.from: when provided, the second argument must be a function');
        }
        if (arguments.length > 2) {
          T = arguments[2];
        }
      }
      var len = toLength(items.length);
      var A = isCallable(C) ? Object(new C(len)) : new Array(len);
      var k = 0;
      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;
      }
      A.length = len;
      return A;
    };
  }());
}