Array.from()

Array.from()  metoda tworzy nową instację tablicy z obiektu podobnego do tablicy lub obiektu iterowalnego.

W ES6, składania klas dopuszcza dziedziczenie obu rodzajów klas: wbudowanych oraz zdefiniowanych przez programistę; w rezultacie, statyczne metody jak Array.from są "dziedziczone" przez klasę pochodnę i tworzą nową instancję klasy pochodnej, a nie klasy Array.

Składnia

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

Argumenty

arrayLike

Obiekt podobny do tablicy lub iterowalny.

mapFn

Opcjonalny. Funkcja mapująca wywoływany z każdym elementem tablicy.

thisArg

Opcjonalny. Wartość używana jako this podczas wykonywania mapFn.

Opis

Array.from() pozwala tworzyć tablicę z:

• obiektu podobnego do tablicy (obiekt z właściwością length property oraz liczbowymi elementami)
• iterable objects (objects where you can get its elements, such as Map and Set).

Array.from() has an optional parameter mapFn, which allows you to execute a map function on each element of the array (or subclass object) that is being created. More clearly, Array.from(obj, mapFn, thisArg) is the same as Array.from(obj).map(mapFn, thisArg), except that it does not create an intermediate array. This is especially important for certain array subclasses, like typed arrays, since the intermediate array would necessarily have values truncated to fit into the appropriate type.

The length property of the from() method is 1.

Examples

// Array-like object (arguments) to Array
function f() {
  return Array.from(arguments);
}

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


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


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


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


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

Polyfill

Array.from was added to the ECMA-262 standard in the 6th edition; as such it may not be present in other implementations of the standard. You can work around this by inserting the following code at the beginning of your scripts, allowing use of Array.from in implementations that don't natively support it.  This algorithm is exactly the one specified in ECMA-262, 6th edition, assuming Object and TypeError have their original values and that callback.call evaluates to the original value of Function.prototype.call. In addition, since true iterables can not be polyfilled, this implementation does not support generic iterables as defined in the 6th edition of ECMA-262.

// 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;
    };
  }());
}

Specifications

Browser compatibility

See also

• Array
• Array.prototype.map()
• TypedArray.from()