HTML:
<form>
<p><input id="subject" name="subject" disabled size="60"></p>
<p><input id="Second_Field" class="part">
<input id="Third_Field" class="part">
<input id="Fourth_Field" class="part"></p>
</form>
</ p>
JavaScript:
var updateSubject = function() {
var outArray = [];
for (var i=0;i<parts.length;i++) {
if (parts[i].value !== '' ) {
outArray.push(parts[i].value);
}
}
document.getElementById('subject').value = outArray.join(' ');
};
var parts = document.getElementsByClassName('part');
for (var i=0;i<parts.length;i++) {
parts[i].onkeydown = updateSubject;
}
</ p>