Extend and Merge (don't overwrite properties that reference objects)

extend_and_merge.js

(function(carrier){

  var hasOwn = Object.prototype.hasOwnProperty;
  var slice  = Array.prototype.slice;

  function extend(obj,src){
    for (var p in src){
      if (hasOwn.call(src,p)) {
        if (p in obj && typeof obj[p] === 'object' && obj[p]) {
          extend(obj[p],src[p]);
        }
        else {
          obj[p] = src[p];
        }
      }
    }
  }

  function merge(){
    var result = {};
    var args = slice.call(arguments,0);
    for (var i=0, len=args.length;i<len;i++){
      extend(result,args[i]); 
    }
    return result;
  }

  carrier.extend = extend;
  carrier.merge  = merge;

})(this);

/* 
var a = {};
var b = {a:'Hello', b:{}};
var c = {a:'Bye', b:{f:{d:2}}, c:1};

var r = merge(a, b, c);

alert(JSON.stringify(r,null,0)); //{"a":"Bye","b":{"f":{"d":2}},"c":1}
*/

hasOwn.call(obj,p) && p in obj ... don't you think the second one is quite redundant? ... typeof obj[p] === 'object' && obj[p] !== null ... could be typeof obj[p] === 'object' && obj[p] ;-)

You are right :)