thenify()... turn an err-first callback into two functions that can suitably be passed to a promise.then(..) and/or .catch(..)

1.js

function somethingThatReturnsAPromise(x,y,z) { .. }

function myCallback(err,v) {
  if (err) {
    console.error(err);
  }
  else {
    console.log(v);
  }
}

// **************

somethingThatReturnsAPromise(1,2,3)
  .then( ...thenify(myCallback) );

somethingThatReturnsAPromise(4,5,6)
  .then( thenify(myCallback)[0] )
  .catch( thenify(myCallback)[1] );
                
var [tf,cf] = thenify(myCallback);
somethingThatReturnsAPromise(7,8,9)
  .then(tf)
  .catch(cf);

2.js

function thenify(errFirstCB) {
  return [
    function tf(v) {
      errFirstCB.call(this,undefined,v);
    },
    function cf(e) {
      errFirstCB.call(this,e);
    }
  ];
}