RXJS: Conditional map or mergeMap

Question

If I have a method that sometimes returns a string and sometimes returns a promise that results in a string how can I chain that in my observable?

Example

function doSomething() {
    if (Math.random() < 0.5) {
        return 'test';
    }

    return new Promise(resolve => resolve('test'));
}

const example = of(undefined).pipe(
    mergeMap(() => doSomething())
);

const subscribe = example.subscribe(val => console.log(val));

I would like to always log "test".

Answer 1

One way of solving the problem would be to make the function passed to mergeMap an async function and to await the return value of doSomething, like this:

const example = of(undefined).pipe(
  mergeMap(async () => await doSomething())
);

You can do this because mergeMap expects the function to return an ObservableInput and a promise is a valid ObservableInput.

Doing so takes advantage of that fact await will accept either a promise or an ordinary value - to which the promise returned by the async function will resolve.