Angular2 component's "this" is undefined when executing callback function

Question

I have a component which calls a service to fetch data from a RESTful endpoint. This service needs to be given a callback function to execute after fetching said data.

The issue is when I try use the callback function to append the data to the existing data in a component's variable, I get a EXCEPTION: TypeError: Cannot read property 'messages' of undefined. Why is this undefined?

TypeScript version: Version 1.8.10

Controller code:

import {Component} from '@angular/core'
import {ApiService} from '...'

@Component({
    ...
})
export class MainComponent {

    private messages: Array<any>;

    constructor(private apiService: ApiService){}

    getMessages(){
        this.apiService.getMessages(gotMessages);
    }

    gotMessages(messagesFromApi){
        messagesFromApi.forEach((m) => {
            this.messages.push(m) // EXCEPTION: TypeError: Cannot read property 'messages' of undefined
        })
    }
}

Answer 1

Use the Function.prototype.bind function:

getMessages() {
    this.apiService.getMessages(this.gotMessages.bind(this));
}

What happens here is that you pass the gotMessages as a callback, when that is being executed the scope is different and so the this is not what you expected.
The bind function returns a new function that is bound to the this you defined.

You can, of course, use an arrow function there as well:

getMessages() {
    this.apiService.getMessages(messages => this.gotMessages(messages));
}

I prefer the bind syntax, but it's up to you.

A third option so to bind the method to begin with:

export class MainComponent {
    getMessages = () => {
        ...
    }
}

Or

export class MainComponent {
    ...

    constructor(private apiService: ApiService) {
        this.getMessages = this.getMessages.bind(this);
    }

    getMessages(){
        this.apiService.getMessages(gotMessages);
    }
}