Is there a method to prevent explicitly passing the context of "this"?

Question

Is there a method to prevent explicitly passing the context of "this"?

Currently, I am in the process of developing a new product and have set up both back-end and front-end projects. For the front-end, I opted to use Angular framework with Typescript. As a newcomer to this language (just a few days old), I have encountered a question regarding callbacks and how to avoid explicit passes with the "this" context. I have come across some resources that I will link for further information.

In the code below, I am creating a wrapper for the HttpClient. The basic idea is that flow control with modals following a plugin architecture (backed by angular routing) works best when complemented with a central delegation using observers and subscribers to broadcast errors like 401 for a graceful re-entry (in my opinion). We won't delve deeper into that topic here, but providing this context may be helpful.

Here is the essence of my code: The Wrapper =>

export class WebService {
  
  constructor(private httpClient: HttpClient,
              private exceptionService<Exception>) { }
 
 public post<T>(url: string, dataToPost: any, callBack: (responseData: T) =>
               void, callBackInstance: any): void {

   this.httpClient.post<T>(url, dataToPost).subscribe(
     (data: T) =>  {
       callBack.call(callBackInstance, data);
     },

     (error: HttpErrorResponse) => {
       this.exceptionService.notify(error);
     }
   );

At this point, I can explicitly manage the "this" context for the callback by using .call(). While I do not mind utilizing this method in your suggestions, it's worth noting that the method requires you to pass in the desired "this" context (callbackInstance). This adds a level of responsibility on the caller of the method that I find unnecessary. To me, a class is somewhat akin to an array with the "this" as an initial displacement. Since I am passing in the callback method, is there a way to inspect that method to derive the appropriate "this"? Perhaps something like: callbackInstance = callback.getRelativeContext(); callBack.call(callBackInstance, data); This would remove the extra parameter, making the method less error-prone for my team to use.

I welcome links to relevant resources, so feel free to share if you can provide a more targeted reference.

Links:

For updating the "this" context

Parameter callbacks

EDIT: Based on the accepted answer, I derived and implemented a test case:

const simpleCallback = (response) => {holder.setValue(response); };
service.post<LoginToken>(Service.LOGIN_URL, '', simpleCallback);

angular typescript

Answer 1

Answer №1

If you find yourself needing to pass the context to the callback function, then the callback itself will depend on that specific context:

function explicitContext(callback, context) {
    const arg = 1;
    callback.call(context, arg);
}

function implicitContext(callback) {
    const arg = 1;
    const someCleverContext = {importantVal: 42, importantFunc: () => {}};
    callback.call(someCleverContext, arg);
}

Consider how the context is used when we actually need access to it in the callback function:

function explicitUsage() {
    const someCleverContext = {importantVal: 42, importantFunc: () => {}};
    const callback = function(arg) {this.importantFunc(arg);}
    explicitContext(callback, someCleverContext);
}

function implicitUsage() {
    const callback = function(arg) {this.importantFunc(arg);}
    implicitContext(callback);
}

In both scenarios, we end up revealing details about the context and imposing some responsibility on the user! Unfortunately, there's no easy way around this if the context must be passed. However, most of the time, passing the context may not be necessary.

export class WebService {

    constructor(
        private httpClient: HttpClient,
        private exceptionService<Exception>)
    { }

    public post<T>(url: string, dataToPost: any, callBack: (responseData: T) => void): void {

        this.httpClient.post<T>(url, dataToPost).subscribe(
            (data: T) => {
                callBack(data);
            },

            (error: HttpErrorResponse) => {
                this.exceptionService.notify(error);
            },
        );
    }
}

This approach allows the client code to focus solely on the responseData, and if a specialized context is required, users have the flexibility to bind it themselves:

function usage() {
    let webService: WebService;
    const simpleCallback = (response) => {console.log(response);} // could also inline
    webService.post('/api', {data: 1}, simpleCallback);

    const cleverContextCallback = function(response) {this.cleverLog(response)};
    const cleverContext = {cleverLog: (data) => console.log(data)};
    const boundCallback = cleverContextCallback.bind(cleverContext);
    webService.post('/api', {data: 1}, boundCallback );
}

However, it's worth mentioning that it's often preferable to just return the observable from your services:

export class WebService {

    constructor(
        private httpClient: HttpClient,
        private exceptionService<Exception>)
    { }

    public post<T>(url: string, dataToPost: any, callBack: (responseData: T) => void): Observable<T> {

        const observable = this.httpClient.post<T>(url, dataToPost);
        
        // More handling logic can be added here based on specific requirements
        
        return observable;
    }
}

By handling errors, closing actions, and other tasks within the service itself, users of the service can concentrate on processing the response data without being burdened by additional concerns.

Answer 2

If you find yourself needing to pass the context to the callback function, then the callback itself will depend on that specific context:

function explicitContext(callback, context) {
    const arg = 1;
    callback.call(context, arg);
}

function implicitContext(callback) {
    const arg = 1;
    const someCleverContext = {importantVal: 42, importantFunc: () => {}};
    callback.call(someCleverContext, arg);
}

Consider how the context is used when we actually need access to it in the callback function:

function explicitUsage() {
    const someCleverContext = {importantVal: 42, importantFunc: () => {}};
    const callback = function(arg) {this.importantFunc(arg);}
    explicitContext(callback, someCleverContext);
}

function implicitUsage() {
    const callback = function(arg) {this.importantFunc(arg);}
    implicitContext(callback);
}

In both scenarios, we end up revealing details about the context and imposing some responsibility on the user! Unfortunately, there's no easy way around this if the context must be passed. However, most of the time, passing the context may not be necessary.

export class WebService {

    constructor(
        private httpClient: HttpClient,
        private exceptionService<Exception>)
    { }

    public post<T>(url: string, dataToPost: any, callBack: (responseData: T) => void): void {

        this.httpClient.post<T>(url, dataToPost).subscribe(
            (data: T) => {
                callBack(data);
            },

            (error: HttpErrorResponse) => {
                this.exceptionService.notify(error);
            },
        );
    }
}

This approach allows the client code to focus solely on the responseData, and if a specialized context is required, users have the flexibility to bind it themselves:

function usage() {
    let webService: WebService;
    const simpleCallback = (response) => {console.log(response);} // could also inline
    webService.post('/api', {data: 1}, simpleCallback);

    const cleverContextCallback = function(response) {this.cleverLog(response)};
    const cleverContext = {cleverLog: (data) => console.log(data)};
    const boundCallback = cleverContextCallback.bind(cleverContext);
    webService.post('/api', {data: 1}, boundCallback );
}

However, it's worth mentioning that it's often preferable to just return the observable from your services:

export class WebService {

    constructor(
        private httpClient: HttpClient,
        private exceptionService<Exception>)
    { }

    public post<T>(url: string, dataToPost: any, callBack: (responseData: T) => void): Observable<T> {

        const observable = this.httpClient.post<T>(url, dataToPost);
        
        // More handling logic can be added here based on specific requirements
        
        return observable;
    }
}

By handling errors, closing actions, and other tasks within the service itself, users of the service can concentrate on processing the response data without being burdened by additional concerns.

Is there a method to prevent explicitly passing the context of "this"?

Answer №1

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