Write au lieu d'alert

<script>
function checkFile(fieldObj){
var FileName  = fieldObj.value;
var FileExt = FileName.substr(FileName.lastIndexOf('.')+1);
var FileSize = fieldObj.files[0].size;
var FileSizeMB = (FileSize/1024).toFixed(2);

if ( (FileName || FileExt != "pdf" && FileExt != "doc" && FileExt != "docx") || FileSize>1024){
var error = FileName+" - (" + FileSizeMB + " Ko)";
alert(error);
return false;}
return true;}
</script>
<input type="file" name="confirm" id="confirm" onchange="checkFile(this)">