我猜你可能obj还有跟深层的嵌套对象，这样不知能否满足？

var obj = {'key1':{'key2':123123},'key333':99999};
var str = 'key1.key2';
var item,i=0;
str = str.split('.');

item = obj;
while(i<str.length){
   item = item[str[i++]];
}
console.log(item);

console.log(obj.key1.key2)
console.log(eval("obj."+str))//不推荐
console.log(obj[str.split(".")[0]][str.split(".")[1]])//蛋疼