Efficient JavaScript Delta-encoding merge method

gistfile1.js

var Entity = function(params) {
	this.merge(Entity.defaults);
	this.merge(params);
};

Entity.defaults = {
	position: [0, 0],
	velocity: [0, 0],
	acceleration: [1, 1]
};

Entity.merge = function(obj1, obj2) {
	var cons = obj1.constructor,
		func = obj1._mergeFunc, props, p, i;

	// Create the merge function if one isn't already defined
	if(!func) {
		func = '';

		// Loop over the first objects default properties and create our merge function
		if((props = cons.defaults)) {
			for(p in props) {
				func += 'if(typeof obj2.'+p+' !== "undefined") {' +
					'obj.'+p+' = obj2.'+p+';' +
				'} ';
			}
		}

		// Create and store the new function
		func = cons._mergeFunc = new Function('obj', 'obj2', func);
	}

	// Run the merge function and return the first object
	return func(obj1, obj2), obj1;
};

Entity.prototype.merge = function(obj) {
	return obj ? Entity.merge(this, obj) : false;
}

sweet! thanks!