ExpressionChangedAfterItHasBeenCheckedError: - Javascript

The ExpressionChangedAfterItHasBeenCheckedError is a common error that occurs in Angular. It often happens when an Angular component updates an expression bound to a view, like a variable or a method return value, after the change detection has already run.

This error message is Angular’s way of ensuring that changes made to a data variable or a model are propagated through the rest of the application in a predictable manner. This is usually a good thing, as it prevents subtle bugs that might otherwise arise when different parts of the application are out of sync.

Causes of ExpressionChangedAfterItHasBeenCheckedError

There are a few common causes of the "ExpressionChangedAfterItHasBeenCheckedError" error:

• Calling a method that modifies a view-bound property in the constructor.
• Updating a view-bound property after it has been checked by Angular.
• Updating a view-bound property inside an event handler.

How to Solve ExpressionChangedAfterItHasBeenCheckedError

Here are some steps that can be taken to solve this error.

1. Use setTimeout()

One way to fix this error is to wrap the expression that's causing the error in a setTimeout() method.

  setTimeout(() => {
    // Update the view-bound property here
  });

2. Use ChangeDetectorRef

Using ChangeDetectorRef is another solution that can help solve this error.

import { Component, ChangeDetectorRef } from '@angular/core';

@Component({
  selector: 'app-example',
  templateUrl: './example.component.html'
})
export class ExampleComponent {
  constructor(private cdr: ChangeDetectorRef) {}

  ngOnInit() {
    // Update the view-bound property here
    this.cdr.detectChanges();
  }
}

3. Use ngZone

ngZone is another option that can be used to solve the "ExpressionChangedAfterItHasBeenCheckedError" error.

import { Component, NgZone } from '@angular/core';

@Component({
  selector: 'app-example',
  templateUrl: './example.component.html'
})
export class ExampleComponent {
  constructor(private ngZone: NgZone) {}

  ngOnInit() {
    this.ngZone.run(() => {
      // Update the view-bound property here
    });
  }
}

Conclusion

The "ExpressionChangedAfterItHasBeenCheckedError" error in Angular can be frustrating to solve. However, by following the solutions outlined in this article, you can overcome this error and keep your Angular application running smoothly.