Get the result of updating a Javascript object while treating it as immutable.

update.js

/**
 * Get the result of updating `obj` with `props`, while treating `obj`
 * as immutable.
 */
var update = function(obj, props){

  var keys = Object.keys(props),
    key,
    changed = false,
    i;

  //if props makes no changes, just return obj
  for (i = 0; i<keys.length; i++){
    key = keys[i];
    if (props[keys] !== obj[keys] || !(key in obj)){
      changed = true;
      break;
    }
  }
  if (!changed){
    return obj;
  }

  //we can't set constructor. so traverse the prototype chain until
  //we
  // a) find something with a constructor, or,
  // b) hit null
  var root = obj;

  var hasOwnProperty = Object.prototype.hasOwnProperty;
  while(root !== null && !hasOwnProperty.call(root, 'constructor')) {
    root = Object.getPrototypeOf(root);
  }

  //our return object will have root as it's prototype
  var result = Object.create(root);

  //if something in obj's prototype chain has overridden root,
  //then apply that change to the return object
  for (key in obj){
    if (root === null || !(key in root && root[key] === obj[key])){
      result[key] = obj[key];
    }
  }

  //finally, copy everything from props into the result
  for (i=0; i<keys.length; i++){
    key = keys[i];
    result[key] = props[key];
  }
  return result;
};