Angular2 HTTP Get Observable - How to wait for result

Question

I am trying to validate a user token by the means of a back-end API. I am brand new to Angular2 in general. My question is, if I want isValidToken to return a boolean value of whether or not the token provided was valid, how can I wait for the HTTP call to complete prior to return in result from isValidToken()?

isValidToken(token: string): boolean {
    var isValidToken: boolean = false;
    this.getIsValidToken(token).subscribe(
        data => {
            isValidToken = data.isValidToken;
            return isValidToken;
        },
        error => {
            return false;
        }
    );
}

getIsValidToken(token: string) {
    return this.http.get(this.validateTokenUrl + '?tokenString=' + token)
        .map(res => res.json());
}

Richard Szalay · Accepted Answer · 2016-09-29 00:36:39Z

1

isValidToken needs to return Observable<boolean> or Promise<boolean>. It can't return the value synchronously because it relies on an asynchronous method for it's result.

It's turtles all the way down.

answered Sep 29, 2016 at 0:36

Richard Szalay

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1 Comment

blgrnboy Over a year ago

Yea, I've been reading that all over. Can you give an example though on how I can use the returned value in another method?

Günter Zöchbauer · Accepted Answer · 2016-09-29 05:43:11Z

isValidToken(token: string): boolean {
    return this.getIsValidToken(token);
}

getIsValidToken(token: string) {
    return this.http.get(this.validateTokenUrl + '?tokenString=' + token)
        .map(res => res.json());
}

then you can use it like

someMethod() {
  this.isValidToken.subscribe(token => {
    if(token) {
      this.success = true;
      // or some other code that should be executed when `token` is `true`
    } else {
      this.success = false;
      // or some other code that should be executed when `token` is `false`
    }
  },
  error => {
      this.success = false;
      // or some other code that should be executed when `token` is `false`
  });
}

You can't go to sync execution from an async call. All you can do is return the observable (or promise) for the caller to be able to subscribe and register a callback to be called when data events are emitted or errors occur.

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