Angular Signals: Fix effect() Injection Context Error

The “effect() can only be used within an injection context” error hits hard when you’re trying to wire up a signal effect in Angular signals inside afterNextRender(). It happens because afterNextRender runs outside Angular’s dependency injection scope, after the component’s constructed. Quick fix: inject the Injector at class level, then wrap your effect() call in runInInjectionContext(this.injector, () => { effect(() => { ... }); })—this restores the context without changing your timing.

Contents

• Understanding the Signal Effect Injection Error
• Angular Injection Contexts Explained
• Why afterNextRender Breaks Effects
• Solution 1: runInInjectionContext
• Solution 2: Pass Injector to effect()
• Do You Even Need afterNextRender for Effects?
• Best Practices and Common Pitfalls
• FAQ

Understanding the Signal Effect Injection Error

Ever tried firing off an effect() right after your Angular signals component renders, only to get slapped with “effect() can only be used within an injection context such as a constructor, a factory function, a field initializer, or a function used with runInInjectionContext”? You’re not alone. This crops up specifically in signal effect setups inside lifecycle hooks like afterNextRender().

At its core, Angular signals—introduced in v16 and refined through v19—rely on a reactive system where effects track signal changes and side-effect code, like logging or API calls. But effects demand Angular’s dependency injection (DI) system to function. Without it, Angular can’t resolve dependencies or track signals properly. Your code example nails the issue:

counter = signal(0);

constructor() {
 afterNextRender(() => {
 effect(() => {
 console.log(`counter value: ${this.counter()}`);
 });
 });
}

increment() {
 this.counter.update(val => val + 1);
}

Here, afterNextRender queues the callback post-DOM paint. By then? No injection context. Boom—error. But why does Angular enforce this? Short answer: signals need DI to link up with the component tree and avoid leaks or stale references.

According to the Angular Architects guide on signals, effects “must be created within an injection context.” Skip that, and you’re toast.

Angular Injection Contexts Explained

Angular’s DI isn’t just for services—it’s the backbone for Angular signals too. An injection context means code running where inject() works: constructors, field initializers, factory functions, or explicitly via runInInjectionContext.

Picture this. Constructor? Check. Class field like mySignal = signal(0)? Yep, that’s an initializer. But callbacks? Async stuff? Nope—outside DI scope. Effects inherit this rule because they might implicitly use injected services or need to register with Angular’s change detection.

In Angular 19+, this got stricter for signal effect creation, as noted in this deep dive on injection contexts. Signals now lean harder on fine-grained reactivity, so DI ensures everything stays wired correctly.

What counts as valid?

• Constructor body
• Field initializers (e.g., injector = inject(Injector))
• Provider factories
• runInInjectionContext(injector, callback)

Miss the mark? Angular throws that exact error to prevent buggy, untrackable effects.

Why afterNextRender Breaks Effects

afterNextRender shines for DOM-heavy init—like grabbing a ViewChild element or kicking off canvas animations. It waits until after the browser paints, dodging layout thrashing. Smart for side effects tied to rendered DOM.

But for pure signal effect logic? Overkill. Effects are lazy—they only run when signals change, and they auto-cleanup on destroy. Slapping one inside afterNextRender feels right if you’re thinking “signals might not be ready yet,” but Angular’s reactivity handles that.

The real culprit: timing. afterNextRender callbacks execute after construction, outside DI. No context, no effect(). A DEV Community post on signals and effects spells it out: “By default, you can only create an effect() within an injection context.”

Run your code? Error city. And it cascades—if your effect touches DOM or services, things get weirder.

Solution 1: runInInjectionContext

Easiest, most reliable fix. Inject Injector once at class level (that’s in-context), then wrap your effect in runInInjectionContext.

Refactored code:

import { Component, signal, effect, Injector, afterNextRender, inject } from '@angular/core';

@Component({
 // ...
})
export class MyComponent {
 counter = signal(0);
 private injector = inject(Injector); // Field initializer = injection context!

 constructor() {
 afterNextRender(() => {
 runInInjectionContext(this.injector, () => {
 effect(() => {
 console.log(`counter value: ${this.counter()}`);
 });
 });
 });
 }

 increment() {
 this.counter.update(val => val + 1);
 }
}

Boom. Works. The runInInjectionContext callback now pretends it’s in DI land. Angular tracks the effect properly, ties it to the component lifecycle, and cleans up on destroy.

This pattern’s gold for any post-render Angular effect init. Pro tip: Keep the injector private—avoids tree-shaking issues.

Solution 2: Pass Injector to effect()

Angular v17+ lets you pass { injector } directly to effect() options. Slick alternative, no wrapper needed.

constructor() {
 afterNextRender(() => {
 effect(() => {
 console.log(`counter value: ${this.counter()}`);
 }, { injector: this.injector }); // Pass it here!
 });
}

Same class-level private injector = inject(Injector);.

Both fixes restore context. Prefer runInInjectionContext for complex callbacks (multiple effects/services). Use the option for one-liners. Angular Architects endorses either.

Do You Even Need afterNextRender for Effects?

Here’s the kicker: probably not. Pure signal-tracking effects? Create them in the constructor or field initializer. They’ll fire on first read (lazy), then react forever.

counter = signal(0);

constructor() {
 effect(() => { // No afterNextRender needed!
 console.log(`counter: ${this.counter()}`);
 });
}

Why delay? Effects don’t block render—they’re async-friendly. Use afterNextRender only if:

• Effect reads DOM (e.g., effect(() => { elementRef.nativeElement.scrollTop = this.scrollPos(); }))
• Third-party libs need painted DOM
• Heavy computation you’d defer post-paint

Otherwise? Skip it. Simpler, no DI hacks. Reactive magic handles the rest.

Best Practices and Common Pitfalls

Nail Angular signals effects like a pro:

1. Default to constructor/field init. Fastest, no extras.
2. Injector once, reuse. Don’t inject(Injector) inside callbacks.
3. Cleanup awareness. Effects auto-destroy on component destroy—trust it.
4. Avoid globals. No effect() in utils/services without explicit context.
5. Test reactivity. Mutate signals, confirm logs fire.

Pitfalls?

• Multiple injectors: Wrong tree = leaks.
• Forgetting destroy: Rare, but async effects might need onCleanup().
• Overusing delays: afterNextRender twice? Nested hell.

Scale it: For directives/services, same rules. In Angular 19+, signals integrate deeper with zoneless—DI stays king.

Real-world? Track form state post-render, sync URL params with DOM selects. But benchmark—premature optimization bites.

FAQ

Q: Can I use effect() in ngAfterViewInit?
A: Same error—ngAfterViewInit is post-construction, no DI. Use runInInjectionContext there too.

Q: What’s better, runInInjectionContext or {injector}?
A: {injector} for simple effects. Wrapper for batches or conditionals.

Q: Effects leak memory?
A: Nope—Angular destroys them on component destroy if created in-context.

Q: Angular 20 changes? (As of 2025-12-29)
A: No major shifts; v19 stabilized this. Check angular.dev for updates.

Q: Non-component effects?
A: Services need manual runInInjectionContext(inject(Injector), () => effect(...)).

Sources

1. Signals in Angular: Building Blocks - Angular Architects
2. Angular 19 Injection Context - Medium
3. Understanding Signals and Effects in Angular - DEV Community

Conclusion

Fixing the signal effect injection error in Angular signals boils down to respecting DI—use runInInjectionContext or {injector: this.injector} inside afterNextRender, but question if you need the delay at all. Constructor effects handle most cases cleanly. Master this, and your reactive apps stay performant, leak-free, and future-proof. Experiment with the code above; tweak for your DOM quirks. Signals? Game-changer. Go build.