js 输入框为空时 disable button

<script type="text/javascript">
function disableButton()
  {
 var a = document.coursesearchForm.keyword.value;
if(ltrim(a)==""||a.length==0){
document.getElementById("coursesubmit").disabled=true;
} else {
document.getElementById("coursesubmit").disabled=false;
}
  }

function ltrim(str) {
        var pattern = new RegExp("^[\\s]+","gi");
        return str.replace(pattern,"");
}

</script>

<@spring.formInput "searchLogForm.keyword" "onblur='disableButton()'"/>

<input type="submit" value="submit" id="coursesubmit" class="submit"/>