augment.js

var augment = function (obj /* args */) {
  var args = Array.prototype.slice.call(arguments, 1);

  for (var i = 0; i < args.length; i += 1) {
    var arg = args[i];
    // Skip non-objects
    if ('object' !== typeof arg) continue;
    
    for (var k in arg) {
      var v = arg[k];
      // Skip keys that are already present
      if (k in obj) continue;
      obj[k] = v;
    }
  }

  return obj;
};

var a = {
  a: 'b',
  c: 'd',
  e: 'f'
};

var b = {
  a: 'dontoverride',
  f: 'g',
  h: 'a'
}

var c = {
  f: 'dontoverride',
  e: 'dontoverride'
}