ES2015+ syntax and the Zscaler Client Connector legacy PAC parser
medium
Zscaler Client Connector evaluates the App Profile and Forwarding Profile PAC files itself. Older versions offered a legacy parser next to the V8-based one; whether the legacy parser accepts ES2015 syntax is undocumented, so a modern-syntax PAC may fail on an old client while browsers run it.
Why it matters
The app profile settings document a “V8 JavaScript based PAC Parser” option and state that Zscaler
Client Connector 4.9 and later for Windows (4.3 and later for macOS) use the V8 parser regardless of
the setting. Before that, the legacy parser could be in use. Zscaler does not document which
JavaScript level the legacy parser supports. This rule is a draft until a lab run with an old client
shows whether let, const, arrow functions or template literals load. Until then, treat the
client like WinHTTP (PAC-K001): write ES5.
How to fix
Write ES5 syntax (var, function expressions, string concatenation) in PAC files that Zscaler Client Connector downloads, or make sure every client runs 4.9+ (Windows) / 4.3+ (macOS).
Examples
Bad
function FindProxyForURL(url, host) {
const internal = [".corp.example", ".lab.example"];
for (const suffix of internal) {
if (dnsDomainIs(host, suffix)) {
return "DIRECT";
}
}
return "PROXY ${GATEWAY}:80; PROXY ${SECONDARY_GATEWAY}:80";
}
Good
function FindProxyForURL(url, host) {
if (dnsDomainIs(host, ".corp.example") || dnsDomainIs(host, ".lab.example")) {
return "DIRECT";
}
return "PROXY ${GATEWAY}:80; PROXY ${SECONDARY_GATEWAY}:80";
}