NG0600: Error thrown when updating signals inside not computed functions

[lucasousi]

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

core

Is this a regression?

No

Description

The error NG0600: Writing to signals is not allowed in a computed is incorrectly triggered even when the signal update occurs outside a computed function.

In the following example, a function is executed within the template, where it updates a signal. However, this triggers the NG0600 error unexpectedly.

https://stackblitz.com/edit/stackblitz-ng0600-error-polygon?file=src%2Fmain.ts

import { Component, signal } from '@angular/core';
import { bootstrapApplication } from '@angular/platform-browser';

@Component({
  selector: 'app-root',
  template: `
    <p>NG0600 occurring with common functions</p>
    <button [disabled]="disabled()">Test</button>

    {{fnInTemplate()}}
  `,
})
export class App {
  protected readonly disabled = signal(true);

  protected fnInTemplate() {
    this.disabled.update((v) => !v);
  }
}

bootstrapApplication(App);

This issue may be related to the #60143

Please provide a link to a minimal reproduction of the bug

https://stackblitz.com/edit/stackblitz-ng0600-error-polygon?file=src%2Fmain.ts

Please provide the exception or error you saw

NG0600: Writing to signals is not allowed in a computed

Workaround

If the signal.set function is executed inside a setTimeout or any kind of ngZone escape, it works as expected.

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

Angular CLI: 19.0.7
Node: 18.20.3
Package Manager: npm 10.2.3
OS: linux x64

Angular: 19.0.6
... animations, common, compiler, compiler-cli, core, forms
... platform-browser, router

Package                      Version
------------------------------------------------------
@angular-devkit/architect    0.1900.7
@angular-devkit/core         19.0.7
@angular-devkit/schematics   19.0.7
@angular/build               19.0.7
@angular/cli                 19.0.7
@schematics/angular          19.0.7
rxjs                         7.8.1
typescript                   5.6.3
zone.js                      0.15.0

Anything else?

No response

[JeanMeche]

The function you're invoking in the template is updating a signal.
As you might know, a template is a reactive context. Hence the error.

We kept #60143 open as we might want to improve the error message to make it more explicit that the update happened in the context of a template execution (and not a computed).

Closing as work as intented.

[lucasousi]

The function you're invoking in the template is updating a signal. As you might know, a template is a reactive context. Hence the error.

We kept #60143 open as we might want to improve the error message to make it more explicit that the update happened in the context of a template execution (and not a computed).

Closing as work as intented.

This is a simplified example derived from an advanced use case. I'm refactoring projects that use functions in templates, where these functions are encapsulated within objects. As part of the transition, I'm using these existing functions to update signals before fully refactoring the code, minimizing breaking changes.

Why it "works" with setTimeouts and so on?

[JeanMeche]

setTimeout creates a new Macrotask and is not executed in the same callstack as the template. It basically becomes its own execution context.
Note that this is a very bad code smell that you have functions that would update signals during template executions.

[lucasousi]

setTimeout creates a new Macrotask and is not executed in the same callstack as the template. It basically becomes its own execution context. Note that this is a very bad code smell that you have functions that would update signals during template executions.

I know, developer life is not so easy. It's an old codebase, hard to create new models and breaking changes.

Thank you @JeanMeche

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