Straight from the Firebase documents:

value: Read and listen for changes to the entire contents of a path.

var ref = firebase.database().ref(users);

firebase.database().ref().on('value', function(snapshot) {

    // Do whatever

});

This function will run once when it is initiated and the once more for every change to the data under the path. snapshot will return all the contents of user/ which is a lot of data to be returned for every change. You can change the ref accordingly.