ExpressionChangedAfterItHasBeenCheckedException: What Developers Need to Know

The Angular framework is the backbone of JavaScript web development, and it offers unparalleled flexibility and scalability. However, even experienced developers can run into annoying errors like the ExpressionChangedAfterItHasBeenCheckedException. This article will break down the common causes of this issue and provide solutions to help you get back to building solid and scalable web applications.

The Anatomy of the ExpressionChangedAfterItHasBeenCheckedException Error

The ExpressionChangedAfterItHasBeenCheckedException error message can be challenging to understand for novices and experienced developers alike. At its core, the error message means that a component’s data changes during the change detection cycle, leading to a cascading effect that can disrupt the stability of your application. The error usually appears during the development or testing phases, as the framework attempts to apply changes to a component.

Most cases of this error are genuine errors, as the Angular framework is doing precisely what it was designed to do. However, it can be frustrating when an error message appears in the middle of testing and debugging a complex application. Thus it’s essential to take preventive measures to prevent or troubleshoot it in case it happens.

Why Does the ExpressionChangedAfterItHasBeenCheckedException Error Happen?

The error occurs when the Angular framework reaches a stage where it can no longer determine the stable state of a component, hence disrupting the change detection process. There are several reasons why this error can occur. Here are some common culprits:

  • 1. Two-way express binding – when a component binding uses a two-way expression binding, any changes to the data within the binding could trigger further changes, disrupting the change detection cycle

  • 2. Asynchronous code processing – when asynchronous operations change the values bound to a component, changes made after the binding declaration will cause the ExpressionChangedAfterItHasBeenCheckedException error to occur.

  • 3. Multiple bindings, including multiple data bindings on one property or complex structures – These multiple bindings can trigger cascading effects that confuse the Angular framework’s change detection process and cause this error.

Solving ExpressionChangedAfterItHasBeenCheckedException Error

If you find yourself stuck with the ExpressionChangedAfterItHasBeenCheckedException error, don’t worry. Here are some ways you can prevent and troubleshoot this issue within your Angular application:

1. Using ChangeDetectionStrategy.OnPush

Angular components use the ChangeDetectionStrategy mechanism to manage change detection. When using ChangeDetectionStrategy.OnPush, change detection executes only when input properties change. This approach helps prevent excesses checks in the change detection process, thereby improving the application’s performance and reducing the likelihood of the ExpressionChangedAfterItHasBeenCheckedException error.

Here’s an example:


@Component({
selector: 'app-todo-list',
templateUrl: './todo-list.component.html',
changeDetection: ChangeDetectionStrategy.OnPush,
})

2. Use ngAfterViewInit Life-Cycle Method

The ngAfterViewInit life-cycle method executes after the Angular framework initializes the child component’s view. Therefore, you can use it to detect changes only after the Angular framework finishes initializing the view, helping to resolve the ExpressionChangedAfterItHasBeenCheckedException error.

Here’s an example:


ngAfterViewInit() {
this.loaderService.isLoading.subscribe((isLoading: boolean) => {
this.isLoading = isLoading;
this.changeDetectorRef.detectChanges();
});
}

3. Use setTimeout Function

The setTimeout function helps defer the execution of code until the Angular change detection process finishes. Therefore, using it can help prevent the ExpressionChangedAfterItHasBeenCheckedException error. Besides, it can help reduce performance issues related to change detection frequency.

Here’s an example:


setTimeout(() => {
this.someData = newValues;
}, 0);

ExpressionChangedAfterItHasBeenCheckedException (FAQs)

What Is the ExpressionChangedAfterItHasBeenCheckedException?

The ExpressionChangedAfterItHasBeenCheckedException is an error that occurs when an Angular component’s data changes during the change detection cycle, leading to a cascading effect that can disrupt the stability of your web application.

What Causes the ExpressionChangedAfterItHasBeenCheckedException?

The most common reasons why this error can occur include two-way express binding, asynchronous code processing, and multiple bindings.

How Can You Fix the ExpressionChangedAfterItHasBeenCheckedException?

You can prevent and troubleshoot the ExpressionChangedAfterItHasBeenCheckedException error using the following techniques:

  • 1. Use ChangeDetectionStrategy.OnPush

  • 2. Use ngAfterViewInit other life-cycle methods.

  • 3. Use the setTimeout function to defer the execution of code.

Is There a Way to Avoid the ExpressionChangedAfterItHasBeenCheckedException Error?

While there is no foolproof way to avoid this error, using the techniques described above can significantly reduce the likelihood of it occurring within your Angular application.

Conclusion

In conclusion, the ExpressionChangedAfterItHasBeenCheckedException error can be frustrating when trying to debug and test Angular web applications. However, by implementing the above strategies, you can help prevent and troubleshoot this error, keeping your web application stable and error-free.

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