Resolve Javascript Promise outside the Promise constructor scope

I have been using ES6 Promise.

Ordinarily, a Promise is constructed and used like this

new Promise(function(resolve, reject){
    if (someCondition){
        resolve();
    } else {
        reject();
    } 
});

But I have been doing something like below to take the resolve outside for the sake of flexibility.

var outsideResolve;
var outsideReject;
new Promise(function(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?