signal forms: keyInParent() returns a string for array items (typed as number) #71616

Which @angular/* package(s) are the source of the bug?

forms

Is this a regression?

No

Description

In signal forms, the type of FieldState.keyInParent() doesn't match what it returns at runtime for array items.

It is declared as Signal<TKey>, where TKey is number for children of an array field, and the JSDoc says:

If the parent field is array-valued, for example, this is the index of this field in that array.

At runtime it returns a string:

const f = form(signal({items: [{n: 'a'}, {n: 'b'}]}));

f.items[1]().keyInParent(); // '1' (string); the type says number

Because the type says number, code like this compiles but never matches:

items.filter((_, i) => i !== item().keyInParent());

The string seems to come from how keys are produced rather than from a deliberate choice: the parent's children map is built from Object.keys(value) (packages/forms/signals/src/field/structure.ts, computeChildrenMap), and the field proxy's get trap receives '1' for f.items[1], so the key is a JS property-key string. FieldNode.keyInParent passes that string through as-is. (FieldContext.index already converts the same key with Number(key).)

Two ways to resolve it:

1. Return a number for children of array fields, which matches the declared type and the JSDoc.
2. Keep the string and change the type (and JSDoc) to string.

I'll open a PR for option 1, since it follows the existing type and docs, but either option removes the mismatch.

This was also noticed in passing in a comment on #67241.

Please provide a link to a minimal reproduction of the bug

Inline above. As a test:

it('keyInParent is a number for array items', () => {
  const f = TestBed.runInInjectionContext(() =>
    form(signal({items: [{n: 'a'}, {n: 'b'}]})),
  );
  expect(f.items[1]().keyInParent()).toBe(1); // fails: received '1'
});

Please provide the exception or error you saw

Expected '1' to be 1.

Please provide the environment you discovered this bug in (run ng version)

@angular/forms 22.2.3
Also reproduced on main (5244ea781d).

Anything else?

No response