A function optionally passed into the NgForOf directive to customize how NgForOf uniquely identifies items in an iterable.

NgForOf needs to uniquely identify items in the iterable to correctly perform DOM updates when items in the iterable are reordered, new items are added, or existing items are removed.

In all of these scenarios it is usually desirable to only update the DOM elements associated with the items affected by the change. This behavior is important to:

• preserve any DOM-specific UI state (like cursor position, focus, text selection) when the iterable is modified
• enable animation of item addition, removal, and iterable reordering
• preserve the value of the <select> element when nested <option> elements are dynamically populated using NgForOf and the bound iterable is updated

A common use for custom trackBy functions is when the model that NgForOf iterates over contains a property with a unique identifier. For example, given a model:

class User {
  id: number;
  name: string;
  ...
}

a custom trackBy function could look like the following:

function userTrackBy(index, user) {
  return user.id;
}

A custom trackBy function must have several properties:

• be idempotent (be without side effects, and always return the same value for a given input)
• return unique value for all unique inputs
• be fast