[javascript] How to loop through key/value object in Javascript?

var user = {};

now I want to create a setUsers() method that takes a key/value pair object and initializes the user variable.

setUsers = function(data) {     
   // loop and init user    
}

where data is like:

234: "john", 23421: "smith", ....

for (var key in data) {
    alert("User " + data[key] + " is #" + key); // "User john is #234"
}

Something like this:

setUsers = function (data) {
    for (k in data) {
        user[k] = data[k];
    }
}