all.js

function all(promises) {
  return promises.reduce(function (previous, next) {
    return join(previous, next, function ([previous_results, next_val]) {
      previous_results.push(next_val);
      return previous_results;
    });
  });
}

function join(x, y) {
  return x.then(function (x_val) {
    y.then(function (y_val) {
      return [x, y];
    })
  })
}