javascript - Access variables declared a component from a RxJS subscribe() function

Question

I am able to use this.variable to access variables in any part of the component, except inside RxJS functions like subscribe() or catch().

In the example below, I want to print a message after running a process:

import {Component, View} from 'angular2/core';

@Component({
    selector: 'navigator'
})
@View({
    template: './app.component.html',
    styles: ['./app.component.css']
})
export class AppComponent {
    message: string;

    constructor() {
        this.message = 'success';
    }

    doSomething() {
        runTheProcess()
        .subscribe(function(location) {
            console.log(this.message);
        });
    }
}

When I run doSomething(), I get undefined. This scenario can be solved using a local variable:

import {Component, View} from 'angular2/core';

@Component({
    selector: 'navigator'
})
@View({
    template: './app.component.html',
    styles: ['./app.component.css']
})
export class AppComponent {
    message: string;

    constructor() {
        this.message = 'success';
    }

    doSomething() {

        // assign it to a local variable
        let message = this.message;

        runTheProcess()
        .subscribe(function(location) {
            console.log(message);
        });
    }
}

I suppose this is related to the this, however, why I can't access the this.message inside the subscribe()?

Answer 1

This has nothing to do with rx or angular, and everything to do with Javascript and Typescript.

I assume you're familiar with the semantics of this in the context of function invocations in Javascript (if not, there are no shortage of explanations online) - those semantics apply in the first snippet, of course, and that's the only reason this.message is undefined inside subscribe()there. That's just Javascript.

Since we're talking about Typescript: Arrow functions are a Typescript construct intended (in part) to sidestep the awkwardness of these semantics by lexically capturing the meaning of this, meaning that this inside an arrow function === this from the outer context.

So, if you replace:

.subscribe(function(location) {
        //this != this from outer context 
        console.log(this.message); //prints 'undefined'
    });

by:

.subscribe((location) => {
     //this == this from the outer context 
        console.log(this.message); //prints 'success'
    });

You'll get your expected result.