if condition cannot catch the null value of my front end username field in node.js and html

i have a username text field in the login.html file i have node.js code as follows

loginbtn.addEventListener("click", function(){  
if(username.value != null){
    socket.emit("login", {username:username.value});
    $("#name").hide();
    $("#mario-chat").show();
}
else{
    err.innerHTML = document.write("please enter username")
 }
});

in the login page i have not entered any name but still my code goes to the if condition and executing the $("#name").hide();$("#mario-chat").show();. please tell me why my if condition is executing rather than else part. thanks in advance.