every javascript object is a simple hashmap which accepts a string or a Symbol as its key, so you could write your code as:
var map = {};
// add a item
map[key1] = value1;
// or remove it
delete map[key1];
// or determine whether a key exists
key1 in map;
javascript object is a real hashmap on its implementation, so the complexity on search is O(1), but there is no dedicated hashcode() function for javascript strings, it is implemented internally by javascript engine (V8, SpiderMonkey, JScript.dll, etc...)
2020 Update:
javascript today supports other datatypes as well: Map and WeakMap. They behave more closely as hash maps than traditional objects.