How to get a unique id for database child in firebase website

I have initialized firebase and want to write into database . I want unique code for every child but i am not able to do that i tried many things but i can't please help me to do this .............. codes are below tell me where i am wrong

<script src="https://www.gstatic.com/firebasejs/4.1.3/firebase.js"></script>

<script>
var rootRef = firebase.database().ref().child('user');
       $('#tex').click(function(){
              rootRef.set({
              emailinfo:$('#emailinfo').val()
       });
});
</script>