KnockoutJS: Access index of item in array from within the JavaScript template

Question

I populate a list from an array using KnockoutJS:

<div data-bind:"foreach: list">
   <input type="text" data-bind="value: myText" />
</div>

function ViewModel() {
    self.list = ko.observableArray([
        new listItem("sample text")
    ]);
};

function listItem (text) {
    this.myText = text;
};

I can assign an id to the individual instances of my input like so

<input data-bind="attr: { id: $index } ...

How do I access this index from within my listItem function? I want to be able to do something like

function listItem (text) {
    this.myText = text;
    this.index = $index;
};

in order to use this for further processing.

Answer 1

You could create a custom binding that sets your property to the index, it would look something like:

ko.bindingHandlers.setIndex = {
    init: function(element, valueAccessor, allBindings, data, context) {
        var prop = valueAccessor();
        data[prop] = context.$index;
    }        
};

This assumes that you are dealing with objects in your array. You would use it like:

<ul data-bind="foreach: items">
    <li data-bind="setIndex: 'myIndex', text: name"></li>
</ul>

So, this copies the $index observable on to your object with the property name that you specify. Sample: http://jsfiddle.net/rniemeyer/zGmcg/

Another way that you can do this outside of bindings (this is how I used to do it before $index) is to subscribe to changes to the observableArray and repopulate an index each time.

Here is what an extension to an observableArray might look like:

//track an index on items in an observableArray
ko.observableArray.fn.indexed = function(prop) {
    prop = prop || 'index';
   //whenever the array changes, make one loop to update the index on each
   this.subscribe(function(newValue) {
       if (newValue) {
           var item;
           for (var i = 0, j = newValue.length; i < j; i++) {
               item = newValue[i];
               if (!ko.isObservable(item[prop])) {
                  item[prop] = ko.observable();
               }
               item[prop](i);      
           }
       }   
   }); 

   //initialize the index
   this.valueHasMutated(); 
   return this;
};

You would then use it like:

this.myItems = ko.observableArray().indexed('myIndexProp');

Here is a sample: http://jsfiddle.net/rniemeyer/bQD2C/