variables in javascript not recognised?

What am i doing wrong

[immols]

How does javascript deal with variables? In this statement 'document.formname.fieldname.value' i cannot use variables?

This does not work
function ajaxFunction(_formname,_fieldname){
//simple code here to set var xmlHttp;
xmlHttp.onreadystatechange=function() {
if(xmlHttp.readyState==4) {
document._formname._fieldname.value=xmlHttp.responseText;
}
}
//open and send statements here
}

This does work
function ajaxFunction(_formname,_fieldname){
//simple code here to set var xmlHttp;
xmlHttp.onreadystatechange=function() {
if(xmlHttp.readyState==4) {
document.contact_name.name.value=xmlHttp.responseText;
}
}
//open and send statements here
}

Checking of values
When i alert(_formname+' '+_fieldname); i do get 'contact_name name' in a popup.

[Fotiman]

Try this method:

document[_formname][_fieldname].value = xmlHttp.responseText;

[immols]

HAH! Brilliant.Thanks a lot! Works like a charm.
Do you know why i works the way you wrote it and not without the brackets?

[quartzy]

thats how you write arrays, in square brackets