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;
};

Can you provide an example of input and output for this function? I don't really understand what it's doing / used for.

Sure thing.

var options = {
  some_value: 6,
  flying_metal_bird: false
};
var parsedOptions = selectKeys(options, {
  some_value: 'someValue',
  flying_metal_bird: 'airplane'
});

/* 
parsedOptions is now: {
  someValue: 6,
  airplane: false
}
*/

or, more to the point of why I wrote it:

var paramsFromRuby = <%= stuff_in_a_format_that_some_JS_code_hates%>;
var options = selectKeys(paramsFromRuby, {
  feet_in_the_air: 'jumping'
});

Oh I see, it's renaming the keys. Maybe this function would be better named "rename-keys"? I got thrown by the use of "parse" and "options"; "parse options" sounds to me like parsing command-line flags.

Yep. Good point. Renaming it to selectKeys, made it select (and not remap) keys if given an array, and if given an object can remap them.

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.