Typescript Nested Observables

Question

Background: Using rxjs Observables

I'm trying to call a method (with an observable) that calls another method. This current setup isn't working for me with the following compilation error in checkIfTrue function in service 1...

A function whose declared type is neither 'void' nor 'any' must return a value.ts(2355)

//component
//makes call to service
process = () => {
     this.service1.checkIfTrue().subscribe(x => {console.log(x)});
};

//service 1
//makes call to service 2
checkIfTrue = (): Observable<boolean> => {
     this.service2.getInt().subscribe(y => { return y == 1;});
};


//service 2
//makes call to service 2
getInt = (): Observable<number> => {
     return Observable.of(1);
};

It seems like I can't return the response from the second method...How can I achieve this?

Christoph Lütjen · Accepted Answer · 2019-06-12 21:52:57Z

1

checIfTrue (): Observable<boolean> => {
     this.service2.getInt().subscribe(y => { return y == 1;});
};

You have two functions here:

checkIfTrue()
y => { return y == 1; }

Your second function has a return statement, your checkIfTrue() has no return statement. That's why it does not return a value. Just as an example, that should make clear why that cannot work:

// y => y == 1 function dummy
function a() {
  return true;
}

// subscribe() dummy
function b(callback: Function) {
  callback();
}

function checkIfTrue() {
  // your subscribe(y => y == 1) line
  b(a);
}

What you're looking for is something like this

checIfTrue (): Observable<boolean> => {
   return this.service2.getInt()
    .pipe(map(y => y == 1));
};

You can shorten this as:

checIfTrue (): Observable<boolean> => this.service2
    .getInt()
    .pipe(map(y => y == 1));

answered Jun 12, 2019 at 21:52

Christoph Lütjen

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2 Comments

pmeyer Over a year ago

Thanks, but you are combining my two methods into one, this is not what I want.... I know how to do it that way! What do you mean by "checkIfTrue has no return statement"

Christoph Lütjen Over a year ago

y => { return y == 1; } is a separate function, it's what I tried to demo with my "a" function. Only this function has a return statement, your checkIfTrue function has no return statement and without a return statement it will not return a value and that's what the compiler error tells you.

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