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Array.from and Array.of · GitHub

Original discussion began on Twitter and can be found by starting here and here. Discussion was continued on es-discuss mailing list in the thread Pure win: Array.from and Array.of

Update Nov. 3, 2011

Official strawman has been posted: http://wiki.ecmascript.org/doku.php?id=strawman:array_extras

Converts a single argument that is an array-like object or list (eg. arguments, NodeList, DOMTokenList (used by classList), NamedNodeMap (used by attributes property)) into a new Array() and returns it;

1. Let O be the result of calling ToObject( arg ).
2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
3. Let len be ToUint32( lenValue ).
4. Let A be a new array created as if by the expression new Array() where Array is the standard built-in constructor with that name.
5. Let k be 0.
6. Repeat, while k < len
   1. Let Pk be ToString( k ).
   2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
   3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString( k ), Property Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
   4. Increase k by 1.
7. return A.

Array.from = function( arg ) {

  var O = Object( arg );
  var len = O.length >>> 0;
  var A = new Array();
  var k = 0;

  while( k < len ) {

    var kValue;

    if ( k in O ) {
      kValue = O[ k ];

      A[ k ] = kValue;
    }

    k++;
  }

  return A;
};

The following use cases are based on the markup in the the attached file domjunk.html.

var divs = document.querySelectorAll("div");

Array.from( divs );
// [ <div class=​"some classes" data-info=​"12">​</div>​, <div data-info=​"10">​</div>​ ]


Array.from( divs ).forEach(function( node ) {
    console.log( node );    
});
// <div class=​"some classes" data-info=​"12">​</div>​
// <div data-info=​"10">​</div>​


Array.from( divs ).filter(function( node ) {
    return !!node.classList.length;
});
// [ <div class="some classes" data-info="12"></div> ]


Array.from( divs ).reduce(function( prev, current ) {
    return ( +prev.dataset.info ) + ( +current.dataset.info );
});
// 22


Array.from( divs[0].classList )
// ["some", "classes"]


// Now shorter then [].foo.call :)
var a = Array;


a.from( divs );
// [ <div class=​"some classes" data-info=​"12">​</div>​, <div data-info=​"10">​</div>​ ]

Array.of provides a constructor that, unlike Array, does not have the special case for new Array(42), which presets length (and hints to implementations to preallocate) but leaves holes in [0, length ).

The use-case is when you can't write a literal, because you are passing a function-that-constructs as a funarg, and the eventual caller may pass only one number arg, or several args. In that case, Array will not do the right thing in the one-number-arg case. Explanation by Brendan Eich

// Harmony/ES.next rest params
Array.of = function( ...args ) { 
  return args; 
};

// Capable today
Array.of = function() { 
  return Array.prototype.slice.call( arguments ); 
};

Array.of( 10 );
// [ 10 ]

// Versus the existing behaviour of new Array():

new Array( 10 );
// [ , , , , , , , , , , ]

// Makes sense when you can't use a literal, such this case...
// ES.next http://wiki.ecmascript.org/doku.php?id=harmony:rest_parameters
var o = (function( ArrayCtor, ...rest ) {
  return ArrayCtor( rest );
})( Array, 10 );

// Using Array.of:
var o = (function( Ctor, args ) {
  return Ctor( ...args );
})( Array.of, args );

// Many args...
Array.of( "things", "that", "aren't", "currently", "an", "array" );

// [ "things", "that", "aren't", "currently", "an", "array" ]

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