Resolving Change Detection Issues: 'Expression has changed after it was checked' in Angular

2024-07-27

In Angular, change detection is a mechanism that keeps the view (HTML template) synchronized with the component's data (TypeScript model).
During change detection, Angular checks for changes in data properties and updates the view accordingly.
In development mode, Angular performs an additional check after each change detection cycle. This extra check ensures that the view hasn't been modified in a way that's outside of Angular's control, potentially leading to an inconsistent UI state.

When the Error Occurs:

This error typically arises when you modify a template expression or component logic in a way that bypasses Angular's change detection mechanism. Here are common scenarios:

Modifying Data in Lifecycle Hooks: If you directly change data properties within lifecycle hooks like ngAfterViewInit or ngOnChanges (without triggering change detection), Angular won't be aware of the update, causing the error.
Asynchronous Operations: If you update data within asynchronous operations (e.g., setTimeout, setInterval, promises, callbacks), the change might occur after Angular's change detection has already finished.
Third-Party Libraries: Certain third-party libraries that directly manipulate the DOM can also trigger this error by modifying the view outside of Angular's control.

Resolving the Error:

Here are some approaches to address the "Expression has changed after it was checked" error:

Use Change Detection Methods:

If you need to update data within a lifecycle hook, employ Angular's built-in change detection methods:
- ChangeDetectorRef.detectChanges(): Manually trigger change detection after making changes (use this cautiously as it can lead to performance issues if used excessively).
- ChangeDetectorRef.markForCheck(): Schedule a change detection cycle for the next tick (preferred approach).
Example:

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

export class MyComponent implements AfterViewInit {
  constructor(private cdRef: ChangeDetectorRef) {}

  ngAfterViewInit() {
    // Perform some asynchronous operation that updates data
    setTimeout(() => {
      this.myData = someUpdatedValue;
      this.cdRef.markForCheck(); // Schedule change detection
    }, 1000);
  }
}

Employ Asynchronous Techniques:

If updates occur due to asynchronous operations, utilize Angular's asynchronous data binding mechanisms:
- async pipe: Streamline handling of asynchronous data in templates.
- Observables: Manage asynchronous data streams effectively.
Example (using async pipe):

import { Observable, of } from 'rxjs';
import { delay } from 'rxjs/operators';

export class MyComponent {
  myData$: Observable<string>;

  constructor() {
    this.myData$ = of('Initial data').pipe(delay(1000)); // Observable with a 1-second delay
  }
}

<p>Data: {{ myData$ | async }}</p>

Isolate Third-Party Library Issues:
- If you suspect a third-party library, try temporarily disabling it or isolating its impact to pinpoint the cause.
- Consider providing a more controlled way for the library to interact with Angular's change detection mechanism.

Key Points:

Understand Angular's change detection cycle.
Employ the correct methods for updating data within lifecycle hooks and asynchronous operations.
Be cautious when using ChangeDetectorRef.detectChanges() due to potential performance implications.
Consider using async pipe or Observables for asynchronous data handling.

Example Codes for "Expression has changed after it was checked" in Angular with TypeScript

Modifying Data in ngAfterViewInit (Without Change Detection):

This code will trigger the error because this.myData is directly changed within ngAfterViewInit without notifying Angular.

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

@Component({
  selector: 'app-my-component',
  template: `
    <p>Data: {{ myData }}</p>
  `
})
export class MyComponent {
  myData: string = 'Initial data';

  ngAfterViewInit() {
    // This will cause the error because change detection isn't triggered
    this.myData = 'Updated data after view is initialized';
  }
}

Solution: Use ChangeDetectorRef.markForCheck() to schedule change detection.

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

@Component({
  selector: 'app-my-component',
  template: `
    <p>Data: {{ myData }}</p>
  `
})
export class MyComponent {
  myData: string = 'Initial data';

  constructor(private cdRef: ChangeDetectorRef) {}

  ngAfterViewInit() {
    // Asynchronous operation (simulate)
    setTimeout(() => {
      this.myData = 'Updated data';
      this.cdRef.markForCheck(); // Schedule change detection
    }, 1000);
  }
}

Asynchronous Operation (Using async Pipe):

This code demonstrates using the async pipe to handle asynchronous data updates gracefully.

import { Component, OnInit } from '@angular/core';
import { Observable, of } from 'rxjs';
import { delay } from 'rxjs/operators';

@Component({
  selector: 'app-my-component',
  template: `
    <p>Data: {{ myData$ | async }}</p>
  `
})
export class MyComponent implements OnInit {
  myData$: Observable<string>;

  ngOnInit() {
    this.myData$ = of('Initial data').pipe(delay(1000)); // Observable with a 1-second delay
  }
}

Potential Issue with Third-Party Library (Hypothetical):

This code snippet illustrates a hypothetical situation where a third-party library might manipulate the DOM directly, causing the error.

import { Component } from '@angular/core';
import { MyLibrary } from 'my-third-party-library'; // Replace with actual library

@Component({
  selector: 'app-my-component',
  template: `
    <p>Data: {{ myData }}</p>
  `
})
export class MyComponent {
  myData: string = 'Initial data';

  constructor() {
    MyLibrary.updateDomElement(document.getElementById('myElement'), 'Updated content'); // Hypothetical example
  }
}

Solution:

Isolate the library's impact to confirm if it's the cause.
Investigate if the library provides a way to interact with Angular's change detection.
Consider alternative libraries that respect Angular's mechanisms.

Leveraging Lifecycle Hooks (Strategically):

ngOnChanges for Input Properties:

If your component receives data through input properties, Angular's ngOnChanges lifecycle hook can be used to react to changes in those properties. You can update your component's internal state within this hook and trigger change detection accordingly.

import { Component, Input, OnChanges, SimpleChanges } from '@angular/core';

@Component({
  selector: 'app-my-component',
  template: `
    <p>Data: {{ myData }}</p>
  `
})
export class MyComponent implements OnChanges {
  @Input() myData: string = 'Initial data';

  ngOnChanges(changes: SimpleChanges) {
    if (changes['myData']) {
      this.myData = changes['myData'].currentValue;
      // Optionally, trigger change detection here (if needed):
      // this.cdRef.markForCheck();
    }
  }

  constructor(private cdRef: ChangeDetectorRef) {}
}

Template Reference Variables (Templating Approach):

For specific scenarios where you need access to DOM elements within the template, consider using template reference variables. This allows you to manipulate the DOM directly within the template without bypassing change detection.

<p #myElement>Data: {{ myData }}</p>

<button (click)="updateData()">Update Data</button>

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

@Component({
  selector: 'app-my-component',
  template: `
    `
})
export class MyComponent {
  @ViewChild('myElement') myElementRef: ElementRef;

  myData: string = 'Initial data';

  updateData() {
    this.myData = 'Updated data';
    this.myElementRef.nativeElement.textContent = this.myData; // Update DOM element directly
  }
}