calling method from Angular2 template results in NG0100: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked

Question

I would like to call a method from the template, but the call results in NG0100.

How shall I do this properly in Angular2 ?

This is the ts file:

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

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
})
export class AppComponent {

    public getRandom(): number {
        return Math.random();
    }
}

This is the template:

<p>Here is a random number: {{ getRandom() }}</p>

This is the minimal demo:

https://stackblitz.com/edit/angular-ivy-yuaqfr?file=src/app/app.component.html

Answer 1

You can use Pipes in Angular, for example:

import { Pipe, PipeTransform } from '@angular/core';
import * as moment from 'moment';

@Pipe({
  name: 'random'
})

export class RandomPipe implements PipeTransform {
  transform(value: any, ...args: any[]): any {
     return (value + Math.random());
  }
}

And in the html:

<p> {{ 'Here is a random number: ' | random }}</p>

You should declare Pipe same as component.

declarations: [
.
.
RandomPipe
],

Answer 2

Angular runs change detection by default whenever something change, if you change the value after Angular hasn't finished running the detection, you will get this error, I think you only see this message in dev environment, not on production.

You can solve this by following these 2 approaches in my opinion:

Changing the detection strategy OnPush, there is a good article about this here

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

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class AppComponent {
  getRandom(): number {
    return Math.random();
  }
}

Or using the lifecycle hook ngOnInit for assigning a value to random

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

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
})
export class AppComponent implements OnInit {
  random: number = 0;

  ngOnInit() {
    this.random = Math.random();
  }

  getRandom(): void {
    this.random = Math.random();
  }
}

HTML

<p>Here is a random number: {{ random }}</p>
<button (click)="getRandom()">Random</button>