PAC technology, what is the back-end server built with?

for example, this PAC file, what is the backend server to use to build

function FindProxyForURL(url, host){
if (dnsDomainIs(host, "google.com")) {
return "PROXY ip.test.com:6666";
}else{
return "DIRECT";
}
}
Mar.29,2021

any backend that can output strings is fine. It's just a text file, but it's usually lightweight with nodejs,

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