Array.prototype.diff()

Sometimes in the day to day you find some problem that is just fun to try to solve without a Google (Stack Overflow) search.

Recently the problem I needed to solve was this:

I have a list of 'feed' items on a page where a long poll is watching for new 'feed' items to add to the DOM. In this case, really, all I want to do is highlight ANY feed item that is newly shown with a green highlight which then fades away.

In this case what I decided to do was 'diff' an array of 'feed' ids that have already been displayed with an array of ids that represents the items coming from the server. With a fixed number of items on both sides, 20, I know that I need to highlight anything that is new. It seemed to me that simply comparing the two arrays gives me the ids that I need to work with.

Here was my solution:


Array.prototype.diff = function( arr ) { 
  return this.filter( 
    function( val ) { 
      return arr.indexOf( val ) < 0; 
    }); 
};

Essentially what we're doing here is taking an Array and filtering it with the value of an element of another Array and spitting a new Array back out that tells me exactly what is different.

By way of example:

var index = [ 14, 15, 16, 21, 28 ],
    newIndex = [ 15, 16, 21, 28, 29 ]; 
    
var diffIndex = index.diff( newIndex ); 

console.log( diffIndex ); 

> [ 29] 

var index2 = [ 1, 2, 3, '<3' ], 
    newIndex2 = [ 1, 2, 3 ]; 
    
diffIndex2 = index2.diff( newIndex2 ); 

console.log( diffIndex2 );

> ['<3']

Did my example have to log out as less than three? Of course it did.

Update - 23 August 2014

A little mea culpa follow up here... I had to write a follow-up post that corrects something I missed here... check it out.