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

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/