Define class attributes within a TypeScript callback function

I have encountered an issue with my method involving a subscription to an event from a pub sub messaging service. The problem arises when I attempt to define a class property within the callback function, only to find that the property returns as undefined. I am aware that the reference to 'this' has changed from the class to the method scope. However, I require access to the class's 'this' property in order to successfully assign the value to the class property 'this.icon' within my callback method. How can I achieve this task?

import { Component, OnInit, OnDestroy } from '@angular/core';
import { Subscription } from 'rxjs/Subscription'
import { Events } from '../shared/messages/events';
import { MessageService } from '../shared/messages/message.service';

export class Component implements OnInit, OnDestroy {

  public icon: string;
  private subscription: Subscription;

  constructor() { this.btnClickSubscribe();}

  private btnClickSubscribe(): void {

     this.subscription = this.messageService
        .subscribe(Events.btnClick, (payload) => {

        this.icon = 'fa-trash-o'; 

        console.log(this.icon) //logs the correct value, 'fa-trash-o' 
        //however, it is only accessible within this context. I require it to be available in the class context     
  });
 }

Answer №1

Asynchronous events can sometimes lead to the initial undefined value of this.icon outside the callback function. For more information on how to handle this in Angular2, check out this helpful link: How do I return the response from an Observable/http/async call in angular2?

If you are passing the icon to a child component using @Input(), consider utilizing ngOnChanges in the child component. This lifecycle hook captures changes to icon and allows you to execute specific logic after the value has been set. Here's an example implementation in the child component:

@Input() icon;

ngOnChanges() {
  if(this.icon) {
    console.log('icon is set, now do something with it!')
  }
}

If you encounter issues with the view, there are solutions available such as using the safe navigation operator. You can find more details on this topic here: Cannot read property "totalPrice" of undefined

For a demonstration, you can visit this Demo link.

I hope this information proves to be helpful! :)

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