Javascript – Knockout.js incredibly slow under semi-large datasets

javascriptknockout.jsperformance

I'm just getting started with Knockout.js (always wanted to try it out, but now I finally have an excuse!) – However, I'm running into some really bad performance problems when binding a table to a relatively small set of data (around 400 rows or so).

In my model, I have the following code:

this.projects = ko.observableArray( [] ); //Bind to empty array at startup

this.loadData = function (data) //Called when AJAX method returns
{
   for(var i = 0; i < data.length; i++)
   {
      this.projects.push(new ResultRow(data[i])); //<-- Bottleneck!
   }
};

The issue is the for loop above takes about 30 seconds or so with around 400 rows. However, if I change the code to:

this.loadData = function (data)
{
   var testArray = []; //<-- Plain ol' Javascript array
   for(var i = 0; i < data.length; i++)
   {
      testArray.push(new ResultRow(data[i]));
   }
};

Then the for loop completes in the blink of an eye. In other words, the push method of Knockout's observableArray object is incredibly slow.

Here is my template:

<tbody data-bind="foreach: projects">
    <tr>
       <td data-bind="text: code"></td>
       <td><a data-bind="projlink: key, text: projname"></td>
       <td data-bind="text: request"></td>
       <td data-bind="text: stage"></td>
       <td data-bind="text: type"></td>
       <td data-bind="text: launch"></td>
       <td><a data-bind="mailto: ownerEmail, text: owner"></a></td>
    </tr>
</tbody>

My Questions:

  1. Is this the right way to bind my data (which comes from an AJAX method) to an observable collection?
  2. I expect push is doing some heavy re-calc every time I call it, such as maybe rebuilding bound DOM objects. Is there a way to either delay this recalc, or perhaps push in all my items at once?

I can add more code if needed, but I'm pretty sure this is what's relevant. For the most part I was just following Knockout tutorials from the site.

UPDATE:

Per the advice below, I've updated my code:

this.loadData = function (data)
{
   var mappedData = $.map(data, function (item) { return new ResultRow(item) });
   this.projects(mappedData);
};

However, this.projects() still takes about 10 seconds for 400 rows. I do admit I'm not sure how fast this would be without Knockout (just adding rows through the DOM), but I have a feeling it would be much faster than 10 seconds.

UPDATE 2:

Per other advice below, I gave jQuery.tmpl a shot (which is natively supported by KnockOut), and this templating engine will draw around 400 rows in just over 3 seconds. This seems like the best approach, short of a solution that would dynamically load in more data as you scroll.

Best Solution

Please see: Knockout.js Performance Gotcha #2 - Manipulating observableArrays

A better pattern is to get a reference to our underlying array, push to it, then call .valueHasMutated(). Now, our subscribers will only receive one notification indicating that the array has changed.

Related Question