partition-promise-parallel.js

const compactSparse = xs => xs.filter(x => typeof x !== 'undefined');

module.exports = function(xs, f) {
  const yea = [];
  const nay = [];
  
  return Promise.all(xs.map(function(x, i) {
    return f(x).then(well => (well ? yea : nay)[i] = x);
  })).then(() => [yea, nay].map(compactSparse));
};