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fix(forms): update Signal Forms nullable field guidance #68653

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68 changes: 36 additions & 32 deletions skills/dev-skills/angular-developer/references/signal-forms.md
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Expand Up @@ -2,7 +2,11 @@

Signal Forms are the recommended approach for handling forms in modern Angular applications (v21+). They provide a reactive, type-safe, and model-driven way to manage form state using Angular Signals.

**CRITICAL**: You MUST use Angular's new Signal Forms API for all form-related functionality. Do NOT use null as a value or type of any fields.
**CRITICAL**: You MUST use Angular's new Signal Forms API for all form-related functionality.

Avoid using `null` for required fields.

For optional fields (including numeric fields), `null` can be used to represent an empty or unset value.

## Imports

Expand Down Expand Up @@ -48,18 +52,18 @@ export class Example {
userModel = signal({
name: '', // CRITICAL: NEVER use null or undefined as initial values
email: '',
age: 0, // Use 0 for numbers, NOT null
age: 0, // Use 0 for required numeric fields; use null for optional numeric fields
address: {
street: '',
city: '',
},
hobbies: [] as string[], // Use [] for arrays, NOT null
});

// WRONG - DO NOT DO THIS:
// WRONG - DO NOT DO THIS for required fields:
// badModel = signal({
// name: null, // ERROR: use '' instead
// age: null, // ERROR: use 0 instead
// name: null, // ERROR: use '' for required string fields
// age: null, // VALID only if the field is optional
// items: null // ERROR: use [] instead
// });

Expand Down Expand Up @@ -188,7 +192,7 @@ Do NOT do this: `<input min="1" [formField]>` or `<input [value]="val" [formFiel
<option value="us">US</option>
</select>

<!-- userForm.name can NOT be nullable, because input does not accept null-->
<!-- Required text inputs should not be nullable -->
<input [formField]="userForm.name" />
```

Expand Down Expand Up @@ -506,32 +510,32 @@ form(

## Common Pitfalls (DO NOT DO THESE)

| Error Scenario | WRONG (Common Mistake) | RIGHT (Correct Way) |
| :--------------------- | :-------------------------------------------- | :---------------------------------------------------------- |
| **Accessing Flags** | `form.field.valid()` | `form.field().valid()` |
| **Accessing value** | `form.field.value()` | `form.field().value()` |
| **Setting value** | `form.field.set(x)` | Update model signal: `this.model.update(...)` |
| **Form root flags** | `form.invalid()` | `form().invalid()` |
| **Double-calling** | `form.field()()` | `form.field().value()` |
| **Rules Context** | `({ touched }) => touched()` | `({ state }) => state.touched()` |
| **Calling Paths** | `applyWhen(p.foo, () => p.foo() === 'x')` | `applyWhen(p.foo, ({ valueOf }) => valueOf(p.foo) === 'x')` |
| **applyWhen args** | `applyWhen(condition, () => {...})` | `applyWhen(path, condition, schemaFn)` - needs 3 args |
| **Array length** | `form.items().length` | `form.items.length` (structural) |
| **Multi-select array** | `<select [formField]="form.tags">` (string[]) | Use checkboxes for array fields |
| **readonly attribute** | `<input readonly [formField]>` | Use `readonly()` rule in schema |
| **min/max attributes** | `<input min="1" max="10">` | Use `min()` and `max()` rules in schema |
| **value binding** | `<input [value]="val">` | Do NOT use `[value]` with `[formField]` |
| **when option** | `pattern(p.x, /.../, {when: ...})` | `when` only works with `required()` |
| **Submit callback** | `submit(form, () => { ... })` | `submit(form, async () => { ... })` |
| **Async params** | `params: s.field` | `params: ({ value }) => value()` |
| **Async onError** | Omitting `onError` | `onError` is REQUIRED in `validateAsync` |
| **resource() API** | `request: signal` | `params: signal` |
| **applyEach args** | `applyEach(s.items, (item, index) => ...)` | `applyEach(s.items, (item) => ...)` |
| **Nested @for** | `$parent.$index` | Use `let outerIndex = $index` |
| **FormState import** | `import { FormState }` | `FormState` does not exist, use `FieldState` |
| **Null in model** | `signal({ name: null })` | `signal({ name: '' })` or `signal({ age: 0 })` |
| **Validate syntax** | `validate(s.field, { value } => ...)` | `validate(s.field, ({ value }) => ...)` |
| **Checkbox Array** | `[formField]="form.tags"` (string[]) | Checkboxes ONLY bind to `boolean` |
| Error Scenario | WRONG (Common Mistake) | RIGHT (Correct Way) |
| :--------------------- | :-------------------------------------------- | :------------------------------------------------------------------------ | ------ |
| **Accessing Flags** | `form.field.valid()` | `form.field().valid()` |
| **Accessing value** | `form.field.value()` | `form.field().value()` |
| **Setting value** | `form.field.set(x)` | Update model signal: `this.model.update(...)` |
| **Form root flags** | `form.invalid()` | `form().invalid()` |
| **Double-calling** | `form.field()()` | `form.field().value()` |
| **Rules Context** | `({ touched }) => touched()` | `({ state }) => state.touched()` |
| **Calling Paths** | `applyWhen(p.foo, () => p.foo() === 'x')` | `applyWhen(p.foo, ({ valueOf }) => valueOf(p.foo) === 'x')` |
| **applyWhen args** | `applyWhen(condition, () => {...})` | `applyWhen(path, condition, schemaFn)` - needs 3 args |
| **Array length** | `form.items().length` | `form.items.length` (structural) |
| **Multi-select array** | `<select [formField]="form.tags">` (string[]) | Use checkboxes for array fields |
| **readonly attribute** | `<input readonly [formField]>` | Use `readonly()` rule in schema |
| **min/max attributes** | `<input min="1" max="10">` | Use `min()` and `max()` rules in schema |
| **value binding** | `<input [value]="val">` | Do NOT use `[value]` with `[formField]` |
| **when option** | `pattern(p.x, /.../, {when: ...})` | `when` only works with `required()` |
| **Submit callback** | `submit(form, () => { ... })` | `submit(form, async () => { ... })` |
| **Async params** | `params: s.field` | `params: ({ value }) => value()` |
| **Async onError** | Omitting `onError` | `onError` is REQUIRED in `validateAsync` |
| **resource() API** | `request: signal` | `params: signal` |
| **applyEach args** | `applyEach(s.items, (item, index) => ...)` | `applyEach(s.items, (item) => ...)` |
| **Nested @for** | `$parent.$index` | Use `let outerIndex = $index` |
| **FormState import** | `import { FormState }` | `FormState` does not exist, use `FieldState` |
| **Null in model** | `signal({ name: null })` | Use `''` for required fields, or `null` for optional fields (e.g. `number | null`) |
| **Validate syntax** | `validate(s.field, { value } => ...)` | `validate(s.field, ({ value }) => ...)` |
| **Checkbox Array** | `[formField]="form.tags"` (string[]) | Checkboxes ONLY bind to `boolean` |

## Big Form Example

Expand Down
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