Wrestling jQuery collections and other array-like objects

Pop, push, slice, splice, shift, and unshift anyone?


JavaScript has a number of enumerable objects that look, smell, and feel like arrays but unfortunately do not have Array in their prototype chain. These objects can be frustrating because we usually want to manipulate them like “true” arrays, yet they do not have the full range of array methods (like pop, push, shift, slice, splice, and unshift) or properties. Three enumerable objects come immediately to mind:

  1. jQuery collections
  2. document.styleSheets
  3. arguments

There are at least two options to manipulate these objects like real arrays: either create a new data structure that is a real array, or manipulate the data in-place. Both approaches use Array.prototype.

Option 1: Convert the object into an array

This is the safest option, especially for ‘special’ objects like document.styleSheets and arguments. Direct manipulation of either of these can have far-reaching and unexpected effects. It is better to make a real array from them, and then if we need to alter the original object, we can do so later. Creating the new array is usually quite simple:

var arg_list = [], stylesheet_list = [];

Array.prototype.push.apply( arg_list, arguments );
Array.prototype.push.apply( stylesheet_list, document.styleSheets );

Both arg_list and stylesheet_list are real arrays that can be maniplated using using each, map, reduce, or any other method supported by the Array prototype.

Option 2: Use Array.prototype to manipulate data in-place.

I have found this useful to manipulate jQuery collections. Here’s an example:

  aryProtoPop     = Array.prototype.pop,
  aryProtoUnshift = Array.prototype.unshift,

  rotateList, $h1List

// BEGIN utility /rotateList/
// Moves the last element of the /list/ to the first element.
// Will do so /count/ times.  If /count/ is not provided,
// will rotate once.
rotateList = function ( arg_list, arg_count ) {
  var count = arg_count === undefined ? 1 : arg_count;

  for ( i = 0; i < count; i++ ) {
    aryProtoUnshift.call( arg_list, aryProtoPop.call( arg_list ));

$h1List = $( 'h1' );
rotateList( $h1List, 3 );

The function, rotateList not only works on regular arrays, but also on array-like objects such as a jQuery collection. We can use Array.prototype to do more things, like join two jQuery collections:

  aryProtoPush = Array.prototype.push,

  addToList, $h1List, $h2List

addToList = function ( tgt_list, src_list ) {
  aryProtoPush.apply( tgt_list, src_list );

$h1List = $( 'h1' );
$h2List = $( 'h2' );

addToList( $h1List, $h2List );

// $h1List now has $h2List added to it.

Remember that manipulating data in-place on array-like objects can be dangerous. jQuery methods, for example, might get corrupted if they rely on cached property values such as list length. Tread lightly and test thoroughly, Padawan.

Stay tuned for a blog post on my favorite technique for making fast, light, and clean JavaScript objects.

Cheers, Mike

Written on December 17, 2015