`alert()`

Debug output from a PAC. Where it goes depends on the engine; remove it from production files.

alert(message)  →  undefined

Not part of the original Netscape API. Engines log it somewhere (or not at all), and an engine without it throws a ReferenceError, which turns the whole evaluation into an error.

Example

alert("PAC evaluated for " + host); // remove before deploying

Engine notes

Engine	Behaviour	Source
chromium	Forwarded to the net log (chrome://net-export).	code, verified 2026-10-04 services/proxy_resolver/proxy_resolver_v8.cc AlertCallback
firefox	Written to the browser console as "PAC-alert: ...".	code, verified 2026-10-04 netwerk/base/ProxyAutoConfig.cpp@3f73c528 PACProxyAlert
pacparser	1.5.1 and newer print to stderr.	code, verified 2026-10-04 src/pacparser.c@v1.5.3 pac_alert
pacparser	1.5.0 and older have no alert(), so calling it throws ReferenceError.	doc, unverified pacparser 1.5.1 release notes

Availability

Derived from each engine's vendored helper library plus its native bindings (see engines).
Function	chromium <=51	chromium >=52	firefox all	pacparser <=1.4.6	pacparser 1.5.0	pacparser >=1.5.1	reference all
`alert`	yes	yes	yes	no	no	yes	yes

Related rules

alert() used in the PAC PAC-K002