Reject a promise within an http.get catch block

Question

I have this asynchronous function which uses http.get inside a promise.

private _getSchema(fileName): any {
    return new Promise((resolve, reject) => {
        this.http.get(fileName)
            .map(this._extractData)
            .catch(this._handleError)
            .subscribe(schema => resolve(schema));
    });
};

Rather than call this._handleError in the catch block, I would like to reject my promise and pass reject the error object. How do I do that?

attempt:

private _getSchema(fileName): any {
    return new Promise((resolve, reject) => {
        this.http.get(fileName)
            .map(this._extractData)
            .catch(err => reject(err))
            .subscribe(schema => resolve(schema));
    });
};

gives error:

Argument of type '(err: any) => void' is not assignable to parameter of type '(err: any, caught: Observable) => ObservableInput<{}>'. Type 'void' is not assignable to type 'ObservableInput<{}>'.

I'm using typescript

Tamas Hegedus Tamas Hegedus · Accepted Answer · 2017-01-19T19:20:26

Promises fit better as http results than observables, so I suggest you to go the promise way:

private _getSchema(fileName): any {
    return this.http.get(fileName).toPromise().then(x => this._extractData(x));
};

The way you tried does not work because Observable.prototype.catch has a different signature (It needs to provide a new observable to continue with). Subscribing to onError is what you were searching for:

private _getSchema(fileName): any {
    return new Promise((resolve, reject) => {
        this.http.get(fileName)
            .map(this._extractData)
            .subscribe(resolve, reject);
    });
};

But this is exactly what observable.toPromise() does.

Reject a promise within an http.get catch block

1 Answers