I think the track argument is not described transparently enough in the official Angular documentation and I had a few questions about it that remained unanswered.
In the following article I tried to show you how the track-expression can be written.
Decision maker:
$index -> if collection doesn't change (
@for(todo of todos; track $index) {})unique id -> if collection changes (
@for(todo of todos; track todo.id) {})object reference -> if collection changes and no unique id (
@for(todo of todos; track todo) {})function -> (
@for(todo of todos; track myTrackByFunction) {})
Hope, I could help you a little bit.