pmap implementation - from https://github.com/joubertnel/build.js/blob/4d16a35c1de03f90993aea519980f3d49ca9a3fd/lib/penumerable.js

gistfile1.js

pmap: function(operation, mapping, then) {
    var ret = new Array(this.length);
    var remainingWork = this.length;
    
    this.forEach(function(x, idx, i) {
      operation.call(null, x, function() {
	var extendedArgs = Array.prototype.slice.call(arguments);
	var mappedVal;

	extendedArgs.push(x);

	mappedVal = mapping.apply(null, extendedArgs);
	ret[idx] = mappedVal;

	remainingWork = remainingWork - 1;
	if ((remainingWork === 0) && then) then.call(null, ret);
      });
    });

    return this;
  }