key selection, remapping

selectKeys.js

var selectKeys = function(hash, keys) {
  var output, prop, index;
  output = {};
  if(!keys) return {};

  if(keys instanceof Array) {
    for(index = 0; index < keys.length; index++) {
      output[keys[index]] = hash[keys[index]];
    }
    return output;
  }

  for(prop in keys) {
    if(keys.hasOwnProperty(prop)) {
      if(typeof keys[prop] === 'boolean') {
        if(keys[prop]) output[prop] = hash[prop];
        // if false, don't copy the property
      } else {
        output[keys[prop]] = hash[prop];
      }
    } 
  }
  return output;
};

Line 4 you can use if (!keys) instead of checking for undefined. That will catch null too.

Nice. Changed it. That's a win for compression, too, since typestrings can't be compressed but variable names can.

Line 1 has a syntax error. You can drop the selectKeys:.

var selectKeys = selectKeys: function(hash, keys) {

That's what I get for copying+pasting out of an object literal.