Utilizing a function within the catchError method

Question

Utilizing a function within the catchError method

A function has been defined to handle errors by opening a MatSnackBar to display the error message whenever a get request encounters an error.

handleError(err: HttpErrorResponse) {
    this.snackBar.open(err.message)
    return throwError(err)
}

When subscribing to an observable in the constructor, this error handling function is passed to the catchError method.

constructor(private snackBack: MatSnackBar) {
...
this._obv$ = this.myService.getTypes().pipe(
  catchError(this.handleError)
)
...

However, when an error occurs, the err parameter passed to the handleError function is correct, but the this.snackBar instance is found to be undefined.

The issue can be resolved by using the handleError function in a different way like below:

this._obv$ = this.myService.getTypes().pipe(
  catchError(err => this.handleError(err)
)

What's causing the difference and why is the snackBar instance turning out to be undefined?

angular typescript error-handling rxjs

Answer 1

Answer №1

The issue at hand

The main problem arises when a function is called on an object, as the object is automatically passed as the function's context (referred to as this). This invocation style is sometimes known as a method rather than a function.

Methods are granted implicit access to the object on which they can operate. However, it is possible to make this relationship explicit.

When using this.handleError, it does not invoke the function but merely passes a reference to it. Consequently, catchError may not recognize that it should call this.handleError(err) instead of handleError(err).

In general, if a function utilizes the this keyword, it is the responsibility of the caller to provide the appropriate this context. While dot notation often simplifies this process, it can lead to unexpected behavior in certain situations.

Proposed solution:

One approach to rectify this issue is by employing a lambda function to handle this step for you. Lambda functions, denoted with the arrow syntax =>, always maintain the expected context from where they are defined. Therefore, in this scenario, this will refer to the intended object.

Another solution involves binding the correct object to the function call, allowing you to execute the function as a method on the bound object. While this workaround may be more reliable in some cases since it permits binding any context to a function, in this instance, it confines the context to the current one.

For example:

constructor(private snackBack: MatSnackBar) {
...
this._obv$ = this.myService.getTypes().pipe(
  catchError(this.handleError.bind(this))
)
...

Answer 2

The issue at hand

The main problem arises when a function is called on an object, as the object is automatically passed as the function's context (referred to as this). This invocation style is sometimes known as a method rather than a function.

Methods are granted implicit access to the object on which they can operate. However, it is possible to make this relationship explicit.

When using this.handleError, it does not invoke the function but merely passes a reference to it. Consequently, catchError may not recognize that it should call this.handleError(err) instead of handleError(err).

In general, if a function utilizes the this keyword, it is the responsibility of the caller to provide the appropriate this context. While dot notation often simplifies this process, it can lead to unexpected behavior in certain situations.

Proposed solution:

One approach to rectify this issue is by employing a lambda function to handle this step for you. Lambda functions, denoted with the arrow syntax =>, always maintain the expected context from where they are defined. Therefore, in this scenario, this will refer to the intended object.

Another solution involves binding the correct object to the function call, allowing you to execute the function as a method on the bound object. While this workaround may be more reliable in some cases since it permits binding any context to a function, in this instance, it confines the context to the current one.

For example:

constructor(private snackBack: MatSnackBar) {
...
this._obv$ = this.myService.getTypes().pipe(
  catchError(this.handleError.bind(this))
)
...

Utilizing a function within the catchError method

Answer №1

The issue at hand

Proposed solution:

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