Array.from()

console.log(Array.from("foo"));
// Expected output: Array ["f", "o", "o"]

console.log(Array.from([1, 2, 3], (x) => x + x));
// Expected output: Array [2, 4, 6]

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

var s = new Set(["foo", window]);
Array.from(s);
// ['foo', window]

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

function f() {
  return Array.from(arguments);
}

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

//        

//  
Array.from([1, 2, 3], (x) => x + x);
// [2, 4, 6]

//   
Array.from({ length: 5 }, (v, k) => k);
// [0, 1, 2, 3, 4]

//   ECMA-262, 6- , 22.1.2.1
// : 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);
    };

    //  length  from  1.
    return function from(arrayLike /*, mapFn, thisArg */) {
      // 1.  C   this.
      var C = this;

      // 2.  items  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.  mapfn  undefined,  mapping  false.
      var mapFn = arguments.length > 1 ? arguments[1] : void undefined;
      var T;
      if (typeof mapFn !== "undefined") {
        // 5. 
        // 5. a.   IsCallable(mapfn)  false,   TypeError.
        if (!isCallable(mapFn)) {
          throw new TypeError(
            "Array.from: when provided, the second argument must be a function",
          );
        }

        // 5. b.  thisArg ,  T  thisArg;   T  undefined.
        if (arguments.length > 2) {
          T = arguments[2];
        }
      }

      // 10.  lenValue  Get(items, "length").
      // 11.  len  ToLength(lenValue).
      var len = toLength(items.length);

      // 13.  IsConstructor(C)  true, 
      // 13. a.  A      [[Construct]]
      //      C   ,    len.
      // 14. a. ,  A  ArrayCreate(len).
      var A = isCallable(C) ? Object(new C(len)) : new Array(len);

      // 16.  k  0.
      var k = 0;
      // 17.  k < len,  ... (  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.  putStatus  Put(A, "length", len, true).
      A.length = len;
      // 20.  A.
      return A;
    };
  })();
}