How to add/insert an item into an ObservableArray at a certain position with Knockout.js

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);

Confederacy

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.

Mccrae

Thanks. Splice worked. For knockout users, ko.utils.arrayIndexOf is also useful if you want to find the index to insert your new item based on an existing item. – Confederacy

This won't work with sparse arrays, at least it didn't for me in chrome. See my answer below – Mustachio

Answer 2

For knockout use

viewModel.SomeItems.unshift(someNewItem);

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

Glaciology

This insert the item at the beginning of the array so doesn't help if you want to insert an item seomwhere in the middle. – Confederacy

I somehow missed the part about inserting it anywhere in the array. Yes this only works if you want to insert into the beginning. Sorry about that. – Glaciology

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');

Mustachio

And if you want to do insertAt rather than setAt, just switch this()[index] = value; to this.splice(index, 0, value);. Thanks Adam! – Macrospore