How to add/insert an item into an ObservableArray

Question

All the knockout examples I have found seem to add a new item to the end of an ObservableArray using something like:

viewModel.SomeItems.push(someNewItem);

This of course places the item at the end of the array.

How to I add an item to the ObservableArray at a certain position?

eg. something like:

viewModel.SomeItems.push(someNewItem, indexToInsertItAt);

Answer 1

You should be able to use the native JavaScript splice method -

viewModel.SomeItems.splice(2,0,someNewItem);

Docs here - https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/splice

Example here (not Knockout specific) - How to insert an item into an array at a specific index?

From the Knockout docs -

For functions that modify the contents of the array, such as push and splice, KO’s methods automatically trigger the dependency tracking mechanism so that all registered listeners are notified of the change, and your UI is automatically updated.

Answer 2

For knockout use

viewModel.SomeItems.unshift(someNewItem);

See also: http://knockoutjs.com/documentation/observableArrays.html

Answer 3

I made this extension function which worked nicely for me. Splice wasn't working for me if I was adding to the end of a sparse array.

ko.observableArray.fn.setAt = function(index, value) {
    this.valueWillMutate();
    this()[index] = value;
    this.valueHasMutated();
}

This even works with:

var a = ko.observableArray(['a', 'b', 'c']);
a.setAt(42, 'the answer');