javascript - Checking the input if its empty

I am pretty new to jQuery so I have a problem with this.
I am trying to check if input is empty by jQuery. I have this script:

$('#submit').click(function(){
  if($.trim($('#username').val()) == ''){
    alert('Input can not be left blank');

  }
});

Thanks for reply.

Answer

Check the length property to determine if it's empty:

if ($.trim($('#username').val()).length > 0) {
// not empty
}

Edit:
Removed white spaces $.trim