Question

I am trying to delete an item from a list. I am using knockout.js with the mapping plugin. My code looks like this:

Serialize to Json

@{ var jsonData = new HtmlString(new JavaScriptSerializer().Serialize(Model));}

Template

<script type="text/html" id="imgsList">
    {{each model.Imgs}}
        <div style="float:left; margin: 10px 10px 10px 0;">
            <div><a href="${Filename}"><img src="${Filename}" style="width:100px;"></img></a></div>
            <div data-bind="click: deleteImage">Delete</div>
        </div>
    {{/each}}
</script>

K.O. JavaScript

<script type="text/javascript">
     $(function() {
        //KO Setup
        var viewModel = { 
            "model": ko.mapping.fromJS(@jsonData),
            "deleteImage" : function(item) {alert(item.Filename + ' deleted.');}
        }

        ko.applyBindings(viewModel);
    });
</script>

The HTML

<div data-bind="template: 'imgsList'"></div>

The Question

Everything works as expected. A list of images shows up with delete buttons, however, when you click a button item.Filename is undefined. Thoughts?

Edit: Taken from the KNockout.js Manual: “When calling your handler, Knockout will supply the current model value as the first parameter. This is particularly useful if you’re rendering some UI for each item in a collection, and you need to know which item’s UI was clicked.”

It appears that I am not getting back the Img object I am expecting. I don’t know what I am getting back!

Answer 1

When you use {{each}} syntax in jQuery Templates, the data context is whatever the overall template is bound against. In your case, that is the entire view model.

A few options:

1- you can use your current code and pass the item that you are “eaching” on to the function like ( http://jsfiddle.net/rniemeyer/qB9tp/1/ ):

<div data-bind="click: function() { $root.deleteImage($value); }">Delete</div>

Using an anomymous function in the data-bind is pretty ugly though. There are better options.

2- you can use the foreach parameter of the template binding, which works with jQuery Templates and is more efficient than {{each}} like ( http://jsfiddle.net/rniemeyer/qB9tp/2/ ):

<script type="text/html" id="imgsList">
    <div style="float:left; margin: 10px 10px 10px 0;">
        <div>
            <a href="${Filename}">${Filename}</a>
        </div>
        <div data-bind="click: $root.deleteImage">Delete</div>
    </div>
</script>

<div data-bind="template: { name: 'imgsList', foreach: model.Imgs }"></div>

Now, the context of the template is the individual image object and calling $root.deleteImage will pass it as the first argument.

3- Since, the jQuery Templates plugin is deprecated and Knockout now supports native templates, you might want to choose removing your dependency on the jQuery Templates plugin. You could still use a named template (just need to replace any jQuery Templates syntax with data-bind attributes) like: http://jsfiddle.net/rniemeyer/qB9tp/3/ or even remove the template and just go with the foreach control-flow binding like: http://jsfiddle.net/rniemeyer/qB9tp/4/

<div data-bind="foreach: model.Imgs">
    <div style="float:left; margin: 10px 10px 10px 0;">
        <div>
            <a data-bind="text: Filename, attr: { href: Filename }"></a>
        </div>
        <div data-bind="click: $root.deleteImage">Delete</div>
    </div>
</div>

4- While I prefer option #3, you could even choose to use event delegation and attach a “live” handler like: http://jsfiddle.net/rniemeyer/qB9tp/5/

$("#main").on("click", ".del", function() {
   var data = ko.dataFor(this);
   viewModel.deleteImage(data); 
});

This can be especially beneficial if you would be attaching a large number of the same handlers via the click binding (like in a grid).