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Angular State Management Pitfall: Mutable vs Immutable Signal Updates

•
2 min read
#angular

Angular Signals are a fantastic addition for building reactive applications, offering a simple and powerful way to manage state. However, there's a subtle but common pitfall when using them with objects or arrays that can leave you scratching your head, wondering why your UI isn't updating.

I've seen this many times, both in my own projects and in others. You set up a function to update some state held in a signal, but your UI remains stubbornly unchanged, even though you're sure everything should be reactive. You might even log the signal's value in an effect and see nothing happening. It feels like the signal has stopped working altogether.

What's actually happening is that you're mutating the existing object or array that the signal holds, instead of providing the signal with a new (immutable) one. Signals, by default, check for reference equality to determine if their value has changed. If you mutate an object in place, its reference remains the same, and the signal thinks nothing has changed, thus skipping any updates.

Let's look at a simple example to illustrate this:

//Template
<div>{{ mySignal() | json }}</div>

//Component class
export class App {

  mySignal = signal<{ name: string, score: number }>({
    name: "Zoaib Khan",
    score: 98
  });

  constructor() {
    setInterval(() => {
      const value = this.mySignal();
      value.score = Math.round(Math.random() * 100);

      // ❌ Mutable update - this doesn't work!
      // This passes the SAME object reference back to set().
      // The signal doesn't detect a change.
      // this.mySignal.set(value);

      // ✅ Immutable update - this works!
      // We create a NEW object with the updated score.
      // The signal sees a new reference and triggers updates.
      this.mySignal.set({ ...value });
    }, 1000);
  }
  
}

In the example above, if you uncomment the line this.mySignal.set(value);, you'll notice the UI doesn't update, even though value.score is changing. This is because value is still a reference to the original object that mySignal holds. When set() is called with the same reference, the signal doesn't consider it a new value.

The solution, shown in the working line, is to always provide a new object (or array) to the signal when you want to update its contents. The spread syntax ({ ...value }) is a common and concise way to do this. It creates a shallow copy of value, allowing you to provide a brand new object reference to set(), which then correctly triggers the signal's reactivity and subsequent UI updates.

The Rule of Thumb

Always use immutable updates when updating object or array state with signals. Whether it's through simple JavaScript techniques like the spread operator ({ ...myObject } or [ ...myArray ]), or by leveraging a library like Immer.js, making sure you're always providing a new reference will save you from this subtle but frustrating pitfall.

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