PAC

A Proxy Auto-Configuration file (PAC file) is a file which contains a function, FindProxyForURL(), which is used by the browser to determine whether requests (including HTTP, HTTPS, and FTP) should go directly to the destination or if they need to be forwarded through a web proxy server.

function FindProxyForURL(url, host) {
  /* ... */
}

ret = FindProxyForURL(url, host)

See Proxy Auto-Configuration (PAC) file for details about how these are used and how to create new ones.