In the exciting world of JavaScript development, we occasionally come across challenges that may at first seem daunting, one of which being the notorious Error%3A ExpressionChangedAfterItHasBeenCheckedError%3A Expression has changed after it was checked. This common yet tricky error can, however, be prevented and resolved by understanding the core concepts and timings of Angular’s change detection mechanism.
[h2]Understanding the Error: ExpressionChangedAfterItHasBeenCheckedError in Angular [/h2]
We begin with an in-depth comprehension of the error’s origins. When Angular runs its change detection cycle, it expects that any data bindings will not modify the application’s state. This assures the template becomes stable following the second pass. If, however, Angular detects a change in value after one cycle, it issues the ExpressionChangedAfterItHasBeenCheckedError.
The Angular change detection mechanism operates two checks — first, changes from model to view are checked; second, the model’s state is verified following the first check. The error will thus occur if the model’s state is changed during these two checks, hinting that the view will not reflect the correct state.
[h2]Preventing and Resolving the Error [/h2]
Since we now understand what causes this error, we can move towards countering it. As previously mentioned, Angular does not appreciate state changes during checks. So, we must avoid creating bindings that may modify the state.
One way to achieve this could be with the help of setTimeout function. This JavaScript function aids in managing timing issues by executing a function or an expression asynchronously, therefore allowing us to bypass the error.
setTimeout(() => {
// place code here
}, 0);
The above snippet will ensure our changes happen outside the current call stack, thus effectively bypassing the change detection mechanism.
Step-by-Step Explanation of Replacing The Code [/h2]
The idea here is to ensure you place your state-changing code within the setTimeout function. This should look something like this:
ngAfterViewChecked() {
setTimeout(() => {
this.expressionHasChangedErrorCausingProperty = 'new value';
}, 0);
}
In the snippet above, the ngAfterViewChecked lifecycle hook is a perfect spot to place our state-changing code, as this hook runs immediately following Angular’s check for changes.
The setTimeout function with a delay of 0 milliseconds tells JavaScript to run the function in the next cycle of the event loop, ideally after Angular has completed checking for changes.
Exploring Other Useful Libraries [/h2>
While the setTimeout technique is effective, its application should be considered as a last resort. One should ideally aim for cleaner solutions, such as utilising zone.js, a very useful library for managing asynchronous operations.
this.zone.runOutsideAngular(() => {
// place code here
});
Through using the above snippet, we can control when Angular checks for changes. Zone.js allows Angular to have better control over asynchronous tasks, providing higher efficiency over manual triggering.
All in all, mitigating any programming error is all about understanding the foundation of the architecture of the script, as well as learning to correctly implement the necessary code. The ExpressionChangedAfterItHasBeenCheckedError is just another common challenge that is easily resolvable once we gain a thorough understanding of how Angular’s change detection system works.
