Misplacing this pointer within the $scope.$on event

Question

Misplacing this pointer within the $scope.$on event

After registering the "$routeChangeSuccessEvent" from AngularJS and setting the callback function, I encountered an issue where I couldn't access my controller's instance using "this". It appears that the current this instance is undefined.

The TypeScript code snippet below showcases the problem:

export class Ctlr {

    static $inject = ["$rootScope","$route"];

    constructor(private $scope: ng.IRootScopeService) {
        this.Scope = $scope;
        this.Title = "";
        //this.Scope.$on("$routeChangeSuccessEvent", this.onRouteChangeStart);
        this.RegisterEvents();
        }
    private RegisterEvents(): void {
        this.Scope.$on("$routeChangeSuccessEvent",(event: ng.IAngularEvent, args: any) => {
        //this is undefined
            console.log(this);
        });
    }
    public Scope: ng.IScope;
    public Title: string;

    public onRouteChangeStart(event: ng.IAngularEvent, args: any) {
        //this is undefined
        this.Title = args.$$route.name);
    }

}

I managed to access the Title property by storing it in a reference variable like so:

 private RegisterEvents(): void {
        var ref = this.Title;
        this.Scope.$on("$routeChangeSuccessEvent",(event: ng.IAngularEvent, args: any) => {
            ref = args.$$route.name;
        });
    }

However, this workaround doesn't update the view as expected. It seems like I'm not capturing the correct reference. Is there a proper solution to this issue with AngularJS events?

I have searched for information on this peculiar behavior but haven't found anything conclusive. Could someone provide guidance or a solution to resolve this issue?

javascript angularjs typescript

Answer 1

Answer №1

When a callback is triggered, the scope changes causing the reference to this to become undefined.

Another example to consider is:

var ref = this.Title;

This actually creates a duplicate of Title since it is a string (primitive type). This is why it did not work as expected. Modifying ref does not update this.Title.

A common workaround for this issue is to initialize the definition as follows:

var vm = this;

...
private RegisterEvents(): void {
    this.Scope.$on("$routeChangeSuccessEvent",(event: ng.IAngularEvent, args: any) => {
    //this is undefined
        console.log(vm);
    });
}

Instead of using this throughout your code, you can use vm. The specific name doesn't matter, what's important is capturing a reference to this in a context where it will be correct within a callback function. This method works because this is an object rather than a primitive type, so it takes a reference instead of a copy.

An alternative approach is to utilize bind, which can be applied to any function to specify what this should refer to. For example:

$scope.$on("SomeEventHere", someCallbackFunction.bind(this));

Ultimately, the choice between these methods is a matter of personal preference, though many tend to favor the var something = this; technique.

Answer 2

When a callback is triggered, the scope changes causing the reference to this to become undefined.

Another example to consider is:

var ref = this.Title;

This actually creates a duplicate of Title since it is a string (primitive type). This is why it did not work as expected. Modifying ref does not update this.Title.

A common workaround for this issue is to initialize the definition as follows:

var vm = this;

...
private RegisterEvents(): void {
    this.Scope.$on("$routeChangeSuccessEvent",(event: ng.IAngularEvent, args: any) => {
    //this is undefined
        console.log(vm);
    });
}

Instead of using this throughout your code, you can use vm. The specific name doesn't matter, what's important is capturing a reference to this in a context where it will be correct within a callback function. This method works because this is an object rather than a primitive type, so it takes a reference instead of a copy.

An alternative approach is to utilize bind, which can be applied to any function to specify what this should refer to. For example:

$scope.$on("SomeEventHere", someCallbackFunction.bind(this));

Ultimately, the choice between these methods is a matter of personal preference, though many tend to favor the var something = this; technique.

Answer 3

Answer №2

The reason for this is that 'this' always refers to its parent, which in this case is the function itself. To clarify, you can achieve the desired behavior by implementing the following code:

private RegisterEvents(): void {
    var reference = this;
    this.Scope.$on("$routeChangeSuccessEvent", (event: ng.IAngularEvent, args: any) => {
        console.log(reference);
    });
}

Answer 4

The reason for this is that 'this' always refers to its parent, which in this case is the function itself. To clarify, you can achieve the desired behavior by implementing the following code:

private RegisterEvents(): void {
    var reference = this;
    this.Scope.$on("$routeChangeSuccessEvent", (event: ng.IAngularEvent, args: any) => {
        console.log(reference);
    });
}

Answer 5

Answer №3

One option is to reassign the this variable like so:

this.Scope.$on("$routeChangeSuccessEvent",this.onRouteChangeStart.bind(this));

Answer 6

One option is to reassign the this variable like so:

this.Scope.$on("$routeChangeSuccessEvent",this.onRouteChangeStart.bind(this));

Answer 7

Answer №4

In a similar vein to the suggestions provided in other solutions, and leveraging TypeScript version 1.7 or higher, you can employ fat arrow syntax as demonstrated below:

$scope.$on('some-event', () => { console.log('this here refers to an actual object'); });
$scope.$watch('foo.bar', (a,b) => { console.log('and here too...'); }
$scope.$on('$routeChangeSuccessEvent', () => { console.log('also here...'); });

However, when passing a reference of a function from the same class, it is necessary to utilize the .bind(this) approach:

$cope.$on('$routeChangeSuccessEvent', this.onRouteChangeStart.bind(this));

Answer 8

In a similar vein to the suggestions provided in other solutions, and leveraging TypeScript version 1.7 or higher, you can employ fat arrow syntax as demonstrated below:

$scope.$on('some-event', () => { console.log('this here refers to an actual object'); });
$scope.$watch('foo.bar', (a,b) => { console.log('and here too...'); }
$scope.$on('$routeChangeSuccessEvent', () => { console.log('also here...'); });

However, when passing a reference of a function from the same class, it is necessary to utilize the .bind(this) approach:

$cope.$on('$routeChangeSuccessEvent', this.onRouteChangeStart.bind(this));

Misplacing this pointer within the $scope.$on event

Answer №1

Answer №2

Answer №3

Answer №4

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