How do I check if a JavaScript function returns a Promise?

Question

Say I have two functions:

function f1() {
    return new Promise<any>((resolve, reject) => {
        resolve(true);
    });
}

function f2() {
}

How do I know if f1 will return Promise and f2 will not?

Answer 1

So it's just about normalizing the output. Then run it through Promise.resolve() and co.

var possiblePromise = f1();
var certainPromise = Promise.resolve(possiblePromise).then(...);

or

var possiblePromises = [f1(), f2()];
Promise.all(possiblePromises).then(values => ...);

But you need to have a basic Idea of what these functions return. For Example: Both examples would fail with

function f3(){
    return [somePromise, someOtherPromise];
}

var arrayOfPromisesOrValues = f3();

//here you'll still deal with an Array of Promises in then();
Promise.resolve(arrayOfPromisesOrValues).then(console.log); 
//neither does this one recognize the nested Promises
Promise.all([ f1(), f2(), f3() ]).then(console.log);

//only these will do:
Promise.all(f3()).then(console.log);
Promise.all([ f1(), f2(), Promise.all(f3()) ]).then(console.log);

You need to know that f3 returns an Array of Promises or values and deal with it. Like you need to know that f1 and f2 return a value or a Promise of a value.

Bottom line: you need to have a basic knowledge of what a function returns to be able to run the result through the proper function to resolve the promises.