Cannot access 'trackBySignal' inside of a track expression. about angular HOT 8 CLOSED

This is working as designed; the trackBy function is not allowed to change, so cannot originate from a template variable.

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So i modified my code to actually change trackBy with setInterval, and it still works:

type TestType = {
  id: number;
  name: string;
};

@Component({
  selector: 'app-root',
  standalone: true,
  template: `
    Current trackBy: {{ trackByNonSignal }}

    <!-- THIS WORKS -->
    @for(item of data; track item[trackByNonSignal]) {
      <p>
      #{{ item.id }} &mdash; {{ item.name }}
      </p>
    }

    <!-- @if(trackBy(); as trackBySignal) {
      @for(item of data; track item[trackBySignal]) {
        <p>
        #{{ item.id }} &mdash; {{ item.name }}
        </p>
      }
    } -->
  `,
})
export class App {
  public data = [
    { id: 123, name: 'test' },
    { id: 456, name: 'test2' },
  ];

  public trackByNonSignal: keyof TestType = 'id';
  public trackBy = signal<keyof TestType>('id');

  constructor() {
    let tick = 0;

    setInterval(() => {
      this.trackByNonSignal = tick % 2 === 0 ? 'id' : 'name';
      tick++;
    }, 1000);
  }
}

Example on stackblitz updated too. Take a look @JoostK

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While that doesn't report any errors, it results in unstable identities which for does not account for.

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I didn't want to change trackBy dynamically, that's not my initial idea.
Maybe allow devs to put something to trackBy from template-variable, and throw an error when its value changes?

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We decided not to support referencing template variables because it results in runtime overhead, as it requires dynamic lookup during change detection.

It is unclear to me what you are trying to achieve, if it isn't for changing the value dynamically.

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I am building universal table, which loads data from server.
Since this table supports different entities with different columns, i must put inside trackBy an existing key from entity.
The only solutions for now:

1. use $index, but perforance will go down with table-sorting etc
2. use class variable, but cmon, why do i have to make proxy-variable?

Example entity interfaces:

type Entity1 = {id: 123};
type Entity2 = {uuid: 'sdf-sdf-sdf'};

type RepeatableResponse<T> = {
results: T[];
primaryKey: string;
pagination: ...;
}

Pseudocode:

ts:

@Input()
set endpoint(endpoint: string) {
this.apiPath.set(endpoint);
}

public response = toSignal(this.responseObs);

private responseObs = computed<Observable<RepetableResponse<T>>>(() => {
return this.someService.getResults(this.apiPath());
});

html:

@if(response(); as response) {
<table>
<tbody>
@for(result of response.results; trackBy[response.primaryKey]) {
...
}
</tbody>
</table>
}

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I see. Yes, that would require referencing a component member.

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