Angular *ngFor trackBy

Posted on Jul 19, 2022

When iterating over an array of objects in Angular, the change detection compares the references of the objects to detect changes.

If the object reference now change too, like when updating local data from the network, Angular will discard the identity of all nested components within the ngFor and re-render the entire list. Even if the objects deeply equal, the change detection will still detect the change.

To help our loop caching components and instead changing their inputs, we can add a trackBy function to the ngFor directive.

  *ngFor="let option of field.options; trackBy: fieldOptionTrackBy"

And in TypeScript, we add identity tracking function:

fieldOptionTrackBy(index: number, option: AssistantStepOption): string {
  return option.identifier;

Background: Angular did reset my mat-checkbox within the ngFor. That lead to a discard of the freshly changed values.