The identity track expression specified in the @for loop caused re-creation of the DOM corresponding to all items. This is a very expensive operation that commonly occurs when working with immutable data structures. For example:
@Component({ template: ` <button (click)="toggleAllDone()">All done!</button> <ul> @for (todo of todos; track todo) { <li>{{todo.task}}</li> } </ul> `,})export class App { todos = [ { id: 0, task: 'understand trackBy', done: false }, { id: 1, task: 'use proper tracking expression', done: false }, ]; toggleAllDone() { this.todos = this.todos.map(todo => ({ ...todo, done: true })); }}
In the provided example, the entire list with all the views (DOM nodes, Angular directives, components, queries, etc.) are re-created (!) after toggling the "done" status of items. Here, a relatively inexpensive binding update to the done property would suffice.
Apart from having a high performance penalty, re-creating the DOM tree results in loss of state in the DOM elements (ex.: focus, text selection, sites loaded in an iframe, etc.).
Fixing the error
Change the tracking expression such that it uniquely identifies an item in a collection, regardless of its object identity. In the discussed example, the correct track expression would use the unique id property (item.id):
@Component({ template: ` <button (click)="toggleAllDone()">All done!</button> <ul> @for (todo of todos; track todo.id) { <li>{{todo.task}}</li> } </ul> `,})export class App { todos = [ { id: 0, task: 'understand trackBy', done: false }, { id: 1, task: 'use proper tracking expression', done: false }, ]; toggleAllDone() { this.todos = this.todos.map(todo => ({ ...todo, done: true })); }}