How do I “check” if user use promise?

Question

I want to check if .then() is called from the user else make the function synchronous, this is the code of my function

var fun = (ms, unit, asy) => {
  var second = 1000,
    minute = second * 60,
    hour = minute * 60,
    day = hour * 24,
    week = day * 7,
    month = week * 4,
    year = day * 365; // or 1000 * 60 * 60 * 24 * 7 * 4 * 12

  if ( asy ) {
    return new Promise(function (fulfill, reject){
      try {
        var converted;
        switch (unit) {
          case 'something': 
            // Do something
            break;
          case 'something_else' // etc etc
        }
        fulfill(converted)

      } catch (err) {
        reject(err)
      }
    });
  } else {
    switch (unit) {
     case 'something': 
        // Do something
        break;
     case 'something_else' // etc etc
     // ...
     }
    }
  }
}

Now it checks if the asy value is true and then make it asynchronous but (if it's possible) I want to make it synchronous as default, as long as the user doesn't call .then() .

Answer 1

This can't be done, when then is called or not, your function is already performed, so you can't go back in time.

It could be possible using the "classic" callback way in async js programming:

function doSomething(arg1, ... , callback)
{
   if(callback !== undefined) {
      // Do async way and resolve with the callback
   } else {
      // Do sync
   }
}

Answer 2

It's impossible for a function to know how its return value is used after it returned. The function has finished (although the IO may still be running in the background) and returned by the time .then() is called.

Keep your return types consistent and always return a Promise if there's a chance an operation could be asynchronous. Promise .then() callbacks are normalized so that the order of execution is guaranteed regardless of whether the Promise itself resolved synchronously or asynchronously.