I'm having trouble understanding why I can't access the properties of a class within a function that has been passed to an Angular

Currently, I have integrated HTML 5 geolocation into an Angular component:

...
    export class AngularComponent {
    ...
        constructor(private db: DatabaseService) {}

        // this function is linked to an HTML button
        logCoords(message, action) {
        navigator.geolocation.getCurrentPosition(this.success, this.error, this.options);
        }

        success(pos) {
            function time() {
            ...
            }
            var crd = pos.coords;
            var lat = crd.latitude
            var long = crd.longitude

            let newCoordinates = {'lat': lat, 'long':long, 'time': time}
            this.db.addLocation(newCoordinates)
        }
    ...
    }
...

I am trying to save the result of the getCurrentPosition Method in indexed db using an Angular service, but unfortunately the component class properties are inaccessible (this.db is null).

Why can't I access this.db within the success function and what possible solutions could help me overcome this issue?

javascriptangulartypescriptgeolocation

Answer №1

An issue has arisen with the following line of code:

navigator.geolocation.getCurrentPosition(this.success, this.error, this.options);

Within the above line, this.success is being passed as a callback without the context being set to it. Therefore, according to JavaScript principles, this will always refer to the window object instead of the component reference you are expecting.
To resolve this issue, you need to establish the context (component instance) before passing this.success. For this purpose, JavaScript provides the bind() function.

Implement the simple fix below -

navigator.geolocation.getCurrentPosition(this.success.bind(this), this.error.bind(this), this.options);

For more information on the bind() function, please visit the link below -

https://developer.mozilla.org/enUS/docs/Web/JavaScript/Reference/Global_objects/Function/bind

Answer №2

One reason is that the context of this is lost when passing the method as a callback. However, there are several ways to address this issue:

logCoords(message, action) {
    // bind context
    navigator.geolocation.getCurrentPosition(this.success.bind(this), this.error.bind(this), this.options);
}

Alternatively,

logCoords(message, action) {
    navigator.geolocation.getCurrentPosition(this.success, this.error, this.options);

    success = (pos) => {} // <= arrow function
    error = () => {}      // <= arrow function
}

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