Effect Safety Validator Skill

Expert guidance for identifying and fixing unsafe Angular effect patterns, preventing infinite loops, and ensuring proper effect lifecycle management.

Critical Effect Patterns

Subscription Cleanup in Effects

CRITICAL: All subscriptions in effects MUST be properly cleaned up

// BAD: Subscription never unsubscribed
effect(() => {
  this.service.getData().subscribe(data => {
    this.$data.set(data);
  });
  // Memory leak! Subscription persists until component destroyed
});

// SAFE: Using onCleanup for proper cleanup
effect(() => {
  const subscription = this.service.getData().subscribe(data => {
    this.$data.set(data);
  });

  return () => subscription.unsubscribe();
}, { manualCleanup: true });

// BETTER: Use takeUntilDestroyed (preferred)
effect(() => {
  this.service.getData()
    .pipe(takeUntilDestroyed(this.destroyRef))
    .subscribe(data => this.$data.set(data));
});

Form Mutations in Effects (Feedback Loop Prevention)

CRITICAL: Form mutations in effects MUST use {emitEvent: false} to prevent infinite loops

// DANGEROUS: Feedback loop
effect(() => {
  const value = this.$selectedValue();
  // When form.patchValue emits, it triggers the effect again = infinite loop
  this.form.patchValue({ field: value });
});

// SAFE: Prevent event emission with {emitEvent: false}
effect(() => {
  const value = this.$selectedValue();
  this.form.patchValue({ field: value }, { emitEvent: false });
});

Anti-Pattern: Observable in Effect

CRITICAL: Don't call observable-returning methods inside effects without subscription

// DANGEROUS: Observable created but not subscribed
effect(() => {
  this.service.getData(); // Observable never executes!
});

// SAFE: Subscribe to observable
effect(() => {
  this.service.getData()
    .pipe(takeUntilDestroyed(this.destroyRef))
    .subscribe(data => this.$data.set(data));
});

// OR: Use toSignal (automatic subscription)
$data = toSignal(
  this.service.getData(),
  { initialValue: [] }
);

Unsafe Constructor Effect Pattern

Pattern: Initializing data in constructor effect

// UNSAFE: Effect in constructor
constructor() {
  effect(() => {
    this.service.getData()
      .pipe(takeUntilDestroyed(this.destroyRef))
      .subscribe(data => this.$data.set(data));
  });
}

// SAFE: Move to field initializer or ngOnInit
// Option 1: Use toSignal (automatic)
$data = toSignal(this.service.getData(), { initialValue: [] });

// Option 2: Initialize in ngOnInit
ngOnInit() {
  this.service.getData()
    .pipe(takeUntilDestroyed(this.destroyRef))
    .subscribe(data => this.$data.set(data));
}

// Option 3: Field initializer with manual effect
$data = signal<Data[]>([]);
private initData = effect(() => {
  this.service.getData()
    .pipe(takeUntilDestroyed(this.destroyRef))
    .subscribe(data => this.$data.set(data));
});

Infinite Loop Patterns

Pattern 1: Signal Mutation in Effect (Self-triggering)

// INFINITE LOOP: Effect mutates signal it depends on
effect(() => {
  const count = this.$count(); // Depends on $count
  this.$count.set(count + 1);  // Mutates $count -> triggers effect again
});

// SAFE: Keep signals and effects separate
effect(() => {
  const count = this.$count();
  this.$displayValue.set(count * 2); // Different signal
});

Pattern 2: Form Value Change Loop

// INFINITE LOOP: Form patchValue triggers valueChanges which triggers effect
effect(() => {
  const value = this.form.get('field')?.value;
  this.$selectedValue.set(value);
});

this.form.valueChanges
  .pipe(takeUntilDestroyed(this.destroyRef))
  .subscribe(value => {
    effect(() => {
      this.form.patchValue({ ...value }); // Circular!
    });
  });

// SAFE: One-way data flow
effect(() => {
  const selected = this.$selectedValue();
  this.form.patchValue({ field: selected }, { emitEvent: false });
});

Pattern 3: Array/Object Mutation Loop

// DANGEROUS: Deep mutations in effects
effect(() => {
  const items = this.$items();
  items.forEach(item => {
    item.updated = true; // Mutates tracked array
  });
  // Change detection may trigger effect again with same array reference
});

// SAFE: Create new array with immutable update
effect(() => {
  const items = this.$items();
  const updated = items.map(item => ({ ...item, updated: true }));
  this.$processedItems.set(updated);
});

Detection Checklist

For each effect, verify:

• No self-referential mutations: Effect depends on signal A, doesn't mutate A
• Proper subscription cleanup: Uses onCleanup or takeUntilDestroyed
• Form mutations safe: All form.patchValue() use {emitEvent: false}
• Observable subscription present: Observable-returning calls are subscribed to
• No multiple effects mutating same signal: Would create potential loops
• Data flow direction clear: Effects flow data in one direction
• No circular dependencies: Signal A -> Effect updates B; not B -> Effect updates A
• Initialization pattern safe: No heavy initialization in constructor effects

Refactoring Unsafe Effects

Unsafe: Constructor Effect with Service

export class DataComponent {
  private readonly $data = signal<Data[]>([]);

  constructor(private service: DataService) {
    // UNSAFE: Effect in constructor
    effect(() => {
      this.service.getData().subscribe(data => {
        this.$data.set(data);
      });
    });
  }
}

Safe: Use toSignal

export class DataComponent {
  protected readonly $data = toSignal(
    inject(DataService).getData(),
    { initialValue: [] }
  );
}

Safe: Use Field Initializer

export class DataComponent {
  private readonly service = inject(DataService);
  private readonly destroyRef = inject(DestroyRef);

  protected readonly $data = signal<Data[]>([]);

  // Initializes after constructor completes
  private initData = effect(() => {
    this.service.getData()
      .pipe(takeUntilDestroyed(this.destroyRef))
      .subscribe(data => this.$data.set(data));
  });
}

Verification and Testing

Static Analysis

# Find effect() declarations
grep -r "effect(" --include="*.ts" client/src/app/

# Find potential subscription leaks
grep -r "\.subscribe(" --include="*.ts" client/src/app/ | grep "effect"

# Find patchValue without {emitEvent: false}
grep -r "patchValue" --include="*.ts" client/src/app/ | grep -v "emitEvent"

Testing Effect Safety

it('should not create infinite loop', () => {
  // Ensure effect completes within reasonable time
  fixture.detectChanges();
  TestBed.flushEffects();

  // If still running after flushEffects, likely infinite loop
  expect(component['$data']()).toBeDefined();
});

it('should cleanup subscriptions', () => {
  const subscription = component['dataSubscription'];
  component.ngOnDestroy();
  expect(subscription.closed).toBe(true);
});

Angular 20 Breaking Changes

See references/angular20-migration.md for detailed patterns with untracked(), queueMicrotask(), and the migration checklist.

Key rule: Effects with change detection triggers (BehaviorSubject, router, form mutations, HTTP subscriptions) now throw NG0101. Wrap side effects in untracked() and signal updates in queueMicrotask().

Examples

User says: "My component has an infinite loop after migration"

1. Search for effect( in the component -- find 3 effects
2. Check each: does it mutate a signal it depends on? -> Effect #2 does: reads $count() and sets $count.set()
3. Fix: separate dependency tracking from mutation, use different signal for output
4. Verify: fixture.detectChanges() + TestBed.flushEffects() completes without timeout

User says: "Getting ObjectUnsubscribedError in production"

1. Find effect with subscription: effect(() => { this.service.getData().subscribe(...) })
2. Check cleanup: no takeUntilDestroyed or onCleanup -> subscription leaks
3. Fix: add .pipe(takeUntilDestroyed(this.destroyRef)) before .subscribe()
4. Verify: component destroy cleans up subscription

User says: "NG0101 error after Angular 20 upgrade"

1. Find effect triggering change detection: this.loading$.next(true) inside effect
2. Fix: wrap in untracked(() => { this.loading$.next(true); })
3. For HTTP subscriptions with signal updates: wrap in queueMicrotask()
4. Verify: no NG0101 in console

Common Issues

Issue	Cause	Solution
Maximum call stack exceeded	Effect mutates the signal it depends on	Use separate signals for input/output of effect
ObjectUnsubscribedError	Subscription outlives component lifecycle	Add takeUntilDestroyed(this.destroyRef)
NG0101: tick is called recursively	Effect triggers change detection (Angular 20)	Wrap side effects in untracked()
Form patchValue triggers infinite loop	Missing {emitEvent: false}	Add {emitEvent: false} to ALL patchValue in effects

See Also

• signal-patterns — General signal/computed/effect patterns, mutation detection, parent-child communication
• test-creation — Testing effects safely (NG0101 avoidance, flushEffects patterns)