When running a callback function, the "this" of an Angular 2 component becomes undefined

Question

When running a callback function, the "this" of an Angular 2 component becomes undefined

One issue I'm facing is with a component that fetches data from a RESTful endpoint using a service, which requires a callback function to be executed after fetching the data.

The problem arises when attempting to use the callback function to append the fetched data to an existing variable in the component. An

EXCEPTION: TypeError: Cannot read property 'messages' of undefined

is encountered. Why is this undefined?

TypeScript version being used: 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
        })
    }
}

javascript arrays typescript angular

Answer 1

Answer №1

To resolve the scope issue, you can utilize the Function.prototype.bind method:

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

By passing the gotMessages as a callback, the execution happens in a different context, causing the this reference to be unexpected. The bind function creates a new bound function with the specified this value.

Another approach is to use an arrow function:

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

The choice between bind and arrow functions is subjective.

An additional option is to bind the method from the start:

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

Alternatively, you can bind the method within the constructor:

export class MainComponent {
    ...

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

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

Answer 2

To resolve the scope issue, you can utilize the Function.prototype.bind method:

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

By passing the gotMessages as a callback, the execution happens in a different context, causing the this reference to be unexpected. The bind function creates a new bound function with the specified this value.

Another approach is to use an arrow function:

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

The choice between bind and arrow functions is subjective.

An additional option is to bind the method from the start:

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

Alternatively, you can bind the method within the constructor:

export class MainComponent {
    ...

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

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

Answer 3

Answer №2

Another approach is to handle it this way

retrieveMessages(apiMessages){
    let self = this // utilizing self as another alternative
    apiMessages.forEach((msg) => {
        self.receivedMessages.push(msg) // or self.receivedMessages.push(msg) - in case self was utilized
    })
}

Answer 4

Another approach is to handle it this way

retrieveMessages(apiMessages){
    let self = this // utilizing self as another alternative
    apiMessages.forEach((msg) => {
        self.receivedMessages.push(msg) // or self.receivedMessages.push(msg) - in case self was utilized
    })
}

Answer 5

Answer №3

When you pass the function reference in getMessages, the correct this context is not maintained.

To resolve this issue, you can use a lambda function which automatically binds the appropriate this context within that anonymous function:

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

Answer 6

When you pass the function reference in getMessages, the correct this context is not maintained.

To resolve this issue, you can use a lambda function which automatically binds the appropriate this context within that anonymous function:

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

Answer 7

Answer №4

Encountered a similar problem but managed to fix it by utilizing () => { } in place of the traditional function()

Answer 8

Encountered a similar problem but managed to fix it by utilizing () => { } in place of the traditional function()

Answer 9

Answer №5

Could you please explain the purpose of this function?

storeMessages = (newMessages) => {
  newMessages.forEach((message) => {
    this.store.push(message)
  })
}

Answer 10

Could you please explain the purpose of this function?

storeMessages = (newMessages) => {
  newMessages.forEach((message) => {
    this.store.push(message)
  })
}

When running a callback function, the "this" of an Angular 2 component becomes undefined

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Similar questions

Troubleshooting `TypeError: document.createRange is not a function` error when testing material ui popper using react-testing-library

Error: Model function not defined as a constructor in TypeScript, mongoose, and express

Why can my JavaScript server code read "2011" but not "20,11" when both are formatted as strings?

Adding a Vue component to HTML using a script tag: A step-by-step guide

Show information in a React Native element | Firebase

What steps can be taken to safeguard data while navigating within the Angular framework?

TSConfig - Automatically Generates a ".js" File for Every ".ts" File

Is it recommended to add the identical library to multiple iFrames?

What is the process for selecting and accessing a DOM element?

Utilize the match() method in JavaScript to target and extract a whole paragraph

encountering a type mismatch error while trying to retrieve data from an array

Error in Leaflet: Uncaught TypeError: layer.addEventParent is not a function in the promise

Exploring the scope in JavaScript functions

Libraries that automatically suggest options for integrating objects described by JSON schema

What is the best way to find the average time in Typescript?

Issue with uploading files on the generated client code

Setting up grunt-contrib-nodeunit to generate JUnit XML output: a step-by-step guide

Error: Module 'react' not found. Please make sure it is installed and correctly imported

What is causing the undefined value for the http used in this function?

JavaScript: Transform an Array of Strings into an Array of Objects