This context does not contain the expected object "this"

I seem to be losing the reference to the "This" object within the helper class in this particular structure, and I'm not sure why.

When looking at the getAll method, the this object is currently pointing to the object housed within the servicesDict array within the main component.

What I am aiming for is for the this object to instead refer to the Entity1Utils class.

export class Entity1Utils {
    public getAll(context) {
        this.buildQueryParams();
        // The usage of "this" points to the object { id: 'entity1', method: this.entity1Utils.getAll }
        // found within servicesDict in ManagementComponent
    }

    private buildQueryParams() {
        // logical code
    }
}


@Component({
    selector: 'app-management',
    templateUrl: './management.component.html',
    styleUrls: ['./management.component.css']
})

export class ManagementComponent implements OnInit {
    private entity1Utils: Entity1Utils;
    private servicesDict: any;

    constructor() {
        this.entity1Utils = new Entity1Utils();

        this.servicesDict = [
            { id: 'entity1', method: this.entity1Utils.getAll }
        ];
    }
}

Answer №1

Typically, the context within a function (represented by this) depends on the object that triggers the method.

foo.doSomething() // 'this' refers to foo inside doSomething
const method = foo.doSomething;
method(); // 'this' is undefined
const obj = { method: foo.doSomething }
obj.method() // 'this' points to obj

When you pass a reference to the actual method, detached from the class instance:

{ method: this.entity1Utils.getAll } // referring to getAll method itself

Subsequently, when it gets invoked elsewhere, it runs as a method on servicesDict:

const service = serviceDict[0];

// Invoking 'getAll' on serviceDict entry, so within
// the method, 'this' now pertains to the serviceDict entry
service.method() 

To resolve this issue, you can convert the method into an arrow function which maintains the current scope:

getAll = context => { ... }

Alternatively, you can create a new anonymous inline arrow function to retain the original scope:

{ method: (...args) => this.entity1Utils.getAll(...args) }

Or explicitly bind the method:

{ method: this.entity1Utils.getAll.bind(this.entity1Utils) }

Answer №2

If you want to ensure proper binding in JavaScript, one option is to utilize the bind method like this:

operation: this.tools.getData.bind(this.tools)

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