Implement assert_difference in QUnit

QUnit is a convenient Javascript testing framework for unit-testing jQuery library, and probably any other Javascript libraries.

These days I found that QUnit lacks of one functionality the like as assert_difference in Rails testing, which is very useful when keeping track on specified property for expected increment.

Traditionally one want to testing if the code respond correctly by incrementing 1 in the length property after performing a remove operation, the following code may be used:

var size_before_remove = Library.length;
Library.remove( something ... );
equals ( size_before_remove - Library.length, 1, "increment" );

I encapsulated this kind of behaviors into a function called difference, as follows:

$.extend( window, {
 difference: function() {
  var subject = arguments[0],
   expect = parseInt( arguments[1] ), expect = ( isNaN( expect ) ) ? 1 : expect,
   fn = arguments[2] || arguments[1];
  
  equals ( 0 - eval( subject ) + (function() {
   fn();
   return eval( subject );
  })(), expect, "assert difference" );
 }
} );

Put it before the test case script and invoke it in the test case as the following ways:

difference( 'object.length', 1, function() {
     // doing something that will change the object.length
}
or
difference( 'object.length', function() { } ) // with expected increment equals 1

Maybe it is the fact that QUnit doesn't need assert_difference due to the chainnability of jQuery library. For example, one can use the following code to testing if the size in one object correctly increment by 1 after inserting one node.

equals ( 0 - $('body > *').size() + $('body').append('<li>hi</li>').children().size(), 1, 'increment' );