Angular 2 Error: Expression has Changed After it was Checked
The infamous ExpressionChangedAfterItHasBeenCheckedError in Angular 2 - a common issue that can leave even the most seasoned developers scratching their heads. This error occurs when the Angular change detection mechanism detects that an expression has changed after it was checked, resulting in a confusing and seemingly inexplicable error.
What’s Happening Behind the Scenes?
When content changes in an Angular application, the view is re-rendered. However, this re-rendering process can sometimes occur before the data has been updated, leading to a mismatch between the view and the data. This is particularly true when dealing with complex data structures, such as arrays or objects.
A Typical Scenario:
Suppose you have a table component that displays a list of items. When the user navigates to a new page, the table component is re-rendered, but the data is not yet updated. As a result, the table component is left with outdated data, causing the ExpressionChangedAfterItHasBeenCheckedError.
Solving the Problem:
One common solution to this problem is to introduce a small delay between the re-rendering of the view and the update of the data. This can be achieved by using the setTimeout() function, which allows you to specify a delay in milliseconds.
Example Code:
import { Component, OnInit } from '@angular/core';
@Component({
selector: 'app-table',
template: `
<div class="member_table" *ngIf="showTableBoolean">
<app-table-ability [comeFromParentTheads]="produceThead" [comeFromPaginLists]="produces"></app-table-ability>
</div>
`
})
export class TableComponent implements OnInit {
showTableBoolean = false;
ngOnInit(): void {
setTimeout(() => {
this.showTableBoolean = true;
}, 0);
}
}
In this example, the showTableBoolean variable is initially set to false, causing the table component to be hidden. However, when the ngOnInit() lifecycle function is called, a setTimeout() function is used to update the showTableBoolean variable to true after a delay of 0 milliseconds. This ensures that the table component is re-rendered only after the data has been updated.
The Principle Behind this Solution:
The key to solving this problem lies in understanding the order of execution of JavaScript code. By introducing a delay between the re-rendering of the view and the update of the data, we can ensure that the data is updated before the view is re-rendered, thus avoiding the ExpressionChangedAfterItHasBeenCheckedError.
Conclusion:
The ExpressionChangedAfterItHasBeenCheckedError in Angular 2 is a common issue that can be solved by introducing a small delay between the re-rendering of the view and the update of the data. By using the setTimeout() function, we can ensure that the data is updated before the view is re-rendered, thus avoiding this error.