How to override method in javascript

		function addmethod(object, methodName, fn) {
			//1
			var old = object[methodName];
			object[methodName] = function() {
				//2
				if(fn.length === arguments.length) {
					return fn.apply(this, arguments);
				}else if(typeof old === 'function') {
					return old.apply(this, arguments);
				}
			};
		}

		var ninjas = {
			values: ['aaa', 'bbb','ccc']
		};

		addmethod(ninjas, 'find', function() {
			return this.values;
		});

		addmethod(ninjas,'find', function(index) {
			return this.values[index];
		});

		console.log(ninjas.find());
		console.log(ninjas.find(1));