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

Question

Say I have two functions:

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

function f2() {
}
         


        

Answer 1

This can't be done.

You can, however, cast any function to a Promise-returning function. This is what I'd do here.

const toPromise = function (f) {
  return function () {
    return new Promise((resolve, reject) => {
      const result = f.apply(null, Array.from(arguments));
      try {
        return result.then(resolve, reject); // promise.
      } catch (e) {
        if (e instanceof TypeError) {
          resolve(result); // resolve naked value.
        } else {
          reject(e); // pass unhandled exception to caller.
        }
      }
    });
  };
};

const f = (x) => x;
const g = (x) => Promise.resolve(x);
const h = (x) => Promise.reject(x);

// Naked value.
toPromise(f)(1).then((x) => {console.log(x)});
// Resolved Promise.
toPromise(g)(2).then((x) => {console.log(x)});
// Rejected Promise.
toPromise(h)(3).catch((x) => {console.log(x)});