Count number of objects in array that match another object

Question

right now I'm doing something like this to match objects in an array:

for (var key in users)
{
if (users[key].userID==session)
{
 //do whatever
}
}

but I need to figure out how many times this matches, if it only matches once, then I want it to trigger the event (where it says "//do whatever")

Answer 1

This quits looking as soon it finds 2 matches

var count= 0;
for(var key in users){
    if(users[key].userID== session)++count;
    if(count== 2) break;
}
if(count== 1){
    //do whatever
}

Answer 2

You could use the array.filter method like this:

users.filter(function(a){return (a.userID==session)}).length == 1;

Although the user will need to be running a modern browser (js 1.6) or the filter method be polyfilled.