How can I access Promise resolution callbacks outside the Promise constructor callback’s scope?

Ordinarily, a Promise is constructed and used like this:

new Promise((resolve, reject) => {
  const obj = new MyEventEmitter();
  obj.onsuccess = (event) => { resolve(event.result); };
  obj.onerror = (event) => { reject(event.error); };
});

But recently, I have been doing something like this to take the resolver outside the executor callback for the sake of flexibility:

let outsideResolve;
let outsideReject;
new Promise((resolve, reject) => {
  outsideResolve = resolve; 
  outsideReject = reject; 
});

And later:

onClick = function() {
  outsideResolve();
}

This works fine, but is there an easier way to do this? If not, is this a good practice?

simple:

var promiseResolve, promiseReject;

var promise = new Promise(function(resolve, reject){
  promiseResolve = resolve;
  promiseReject = reject;
});

promiseResolve();