What is "track by" in AngularJS and how does it work?

You should use track by only if you need to go against the default behaviour of ng-repeat which is to remove duplicate items.
You can track the items using the scope property $index or specifying a custom function.

For instance:

<div ng-repeat="x in [42, 42, 43, 43] track by $index">
  {{x}}
</div>

Display all values of the array (42 is displayed twice).

For reference: https://docs.angularjs.org/api/ng/directive/ngRepeat

When you add track by you basically tell angular to generate a single DOM element per data object in the given collection.

You can track by $index if your data source has duplicate identifiers.

If you do need to repeat duplicate items, you can substitute the default tracking behavior with your own using the track by expression.

Example:

[{id:1,name:'one'}, {id:1,name:'one too'}, {id:2,name:'two'}]

Try to use the duplicate values in ng-repeat, you will get an error such as:

Error: ngRepeat:dupes Duplicate Key in Repeater

To avoid this kind of problems you should use track by $index. For example:

<ul>
   <li ng-repeat="item in [1, 2, 3, 3] track by $index">
       {{ item }}
   </li>
</ul>

Here is how you would get $index in nested ng-repeat:

<div ng-repeat="row in matrix">
    <div ng-repeat="column in row">
      <span>outer: {{$parent.$index}} inner: {{$index}}</span>
    </div>
</div>

---

Here are some resources that may help you:

• track by $index documentation
• ngRepeat documentation
• 2014 codelord.net article about ng-repeat performance and track by

Using track by to track strings & duplicate values

Normally ng-repeat tracks each item by the item itself. For the given array objs = [ 'one', 'one', 2, 'five', 'string', 'foo'], ng-repeat attempts to track changes by each obj in the ng-repeat="obj in objs". The problem is that we have duplicate values and angular will throw an error. One way to solve that is to have angular track the objects by other means. For strings, track by $index is a good solution as you really haven't other means to track a string.

track by & triggering a digest & input focuses

You allude to the fact you're somewhat new to angular. A digest cycle occurs when angular performs an exhaustive check of each watched property in order to reflect any change to the correspodant view; often during a digest cycle it happens that your code modify other watched properties so the procedure needs to be performed again until angular detects no more changes.

For example: You click a button to update a model via ng-click, then you do somethings (i mean, the things you wrote in the callback to perform when an user makes a click), then angular trigger digest cycle in order to refresh the view. I'm not too articulate in explaining that so you should investigate further if that didn't clarify things.

So back to track by. Let's use an example:

1. call a service to return an array of objects
2. update an object within the array and save object
3. after save service, depending on what the API returns, you may:
   1. replace the whole object OR
   2. update a value on the existing object
4. reflect change in ng-repeat UI

How you track this object will determine how the UI reflects the change.

One of the most annoying UXs I've experienced is this. Say you have a table of objects, each cell has an input where you want to in-line edit those objects' properties. I want to change the value, then on-blur, save that object while moving to the next cell to edit while you might be waiting on the response. So this is an autosave type thing. Depending on how you setup your track by statement, you may lose current focus (e.g. the field you're currently editing) when the response gets written back into your array of objects.