Promise-Aware Compose

gistfile1.js

function pcompose() {
  var step1;
  var step2;
  var tailArgs;
  // TODO: Handle the case where only one argument is passed.
  if (arguments.length === 2) {
    step1 = arguments[1];
    step2 = arguments[0];
    return function(items) {
      var value = step1(items);
      if (value.then) {
        return value
          .then(function(result) {
            return step2(result);
          });
      } else {
        return step2(value);
      }
    }
  } else {
    tailArgs = Array.prototype.slice.call(arguments, 1);
    return pcompose(arguments[0], pcompose.apply(null, tailArgs));
  }
};