Trim Empty Space Using JavaScript When doing Validation

function ValidateImage() {

   var fileName = document.getElementById('FUHeaderLogo').value;

   if (trim(fileName) == '') {

   alert("Please browse PNG Image");

   return false;

   }

   else {

   return true;

  }

}

function trim(s) {

    var l = 0; var r = s.length - 1;

    while (l < s.length && s[l] == ' ')

    { l++; }

    while (r > l && s[r] == ' ')

    { r -= 1; }

    return s.substring(l, r + 1);

 }