There can be multiple solution for it. we have onKeyUp onKeyDown events for textbox. all you have to do is check if textbox is empty. if so then disable the button else enable the button.
<input type="text" id="txtSomeTextbox">
<input type="button" id="txtSomeButton">
$(document).ready(function() {
$('#txtSomeButton').prop('disabled', true);
$('#txtSomeTextbox').keyup(function() {
if($(this).val() != '') {
$('#txtSomeButton').prop('disabled', false);
}
});
});