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I'm working on an Angular app. I intend to build a...

Created on: March 28, 2025

Answered using O1 by Chat01

Question

I'm working on an Angular app. I intend to build an autocomplete component which will most like be used in cascade manner, i.e. multiple autocompletes, selection change in first loads new options in the second and so on. Basically the autocomplete will have two main inputs, 'items' and 'selectedItem'. I have some questions related to UX.

When the input 'items' list changes should the selectedItem reset?
What is the best way of resetting the sellected item? Should the autocomplete component handle this automatically when the input 'items' list changes or should the parent be in charge of changing the autocomplete's 'selectedItem' to undefined?
Basically I'm trying to plan the implementation of the autocomplete component so I don't needlessly over complicate it

Thought

Handling autocomplete dynamics in Angular, interesting how resetting 'selectedItem' can be integrated with parent control, allowing precise updates to the item list.

Reasoned about autocomplete component behavior for a couple of seconds

Answer

When dealing with a cascaded autocomplete flow, it’s typically clearer and more maintainable to let the parent component “drive” the selections, rather than letting the autocomplete component itself secretly discard or reset values. In other words:

  1. Don’t automatically reset within the child: If your items list changes and the previously selected item no longer makes sense, the parent component should detect that state change and explicitly set selectedItem to undefined (or whatever “no selection” value you’re using). That way the logic is visible in your parent, which “knows” the higher-level context about what changed and why.

  2. Why parent-driven resets are better:

    • Clarity: You avoid “hidden” or implicit resets. When looking at the parent’s logic, you can see exactly when and why the selected item is being cleared.
    • Flexibility: Sometimes you don’t want to reset even if the list changes (e.g., the new items list might be a superset that still includes the old item). By letting the parent handle this, you can easily add that logic without modifying the child component.
    • Predictability: If the autocomplete starts resetting itself automatically, you can end up with race conditions or confusion about what triggered the reset. Having the parent manage selectedItem simplifies that.

  3. Implementation tips:

    • Input bindings: Your autocomplete component might look like: 
      ts
      @Input() items: T[] = [];
@Input() selectedItem?: T;
@Output() selectedItemChange = new EventEmitter<T | undefined>();
    • Event emission: When a user selects an item in your autocomplete, the component emits selectedItemChange. The parent listens and updates its local state. On the parent side, if it detects that items changed in a way that invalidates selectedItem, it can just do selectedItem = undefined.
    • Internal checks: It may be convenient to allow the child component to do a small check like “If the newly assigned selectedItem is not in items, discard it,” but only do that if it’s purely a defensive measure. The main reset should come from the parent.

Overall, letting the parent component handle resetting the selection in response to item list changes keeps the autocomplete’s contract simple. The autocomplete’s job is to render a list of suggestions, highlight the current selectedItem, and emit events when the user changes their selection. The parent is in the best position to coordinate which child’s selection gets reset and when.

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