Little promisify helper

promisifyExample.js

let promisify = (fn) => (...args) =>
  new Promise((rCB, eCB) => 
    fn(...args, (e, r) => {
      if (e)
        eCB(e);
      else
        rCB(r);
    }));

promisify(sass.render({
  file: 'myFile.scss'
))
.then(r => promisify(mkdir)(destPathFolder).then(x => r.css))
.then(css => promisify(fs.writeFile)(destPath, css))
.then(d => console.log('All Done!'))
.catch(e => console.error(`Proccess error: ${e}`));

Found a flaw, if you provide the call back when calling the promisified function it will still return a promise. However the promise will never resolve, instead the callback will run like it wasn't promisified in the first place.

var test = promisify(fs.writeFile);

var p = test('dest.css', 'some css', (e, r) => {
  // I'm called
});

// p is a promise here
p.then(r => {
   // never called
});

I guess you could check the last argument passed in to see if its a function and make assumptions based on that but that seems painful.