Make the PAC Faster Without Changing Its Answers

Preserve overlapping exceptions and fallback order while reducing unnecessary PAC evaluation work.

“Put the common case first” is incomplete advice for a PAC file. The first return ends the decision. If rules overlap, their order is part of the route policy.

Consider a private proxy for one service and a direct route for the rest of its domain:

function FindProxyForURL(url, host) {
    host = host.toLowerCase();
    if (host === 'restricted.example.net') {
        return 'PROXY proxy.example.net:8080';
    }
    if (dnsDomainIs(host, '.example.net')) {
        return 'DIRECT';
    }
    return 'PROXY proxy.example.net:8080';
}

The names are examples. Moving the broad suffix check above the exact-host exception sends the restricted service direct. The script may be shorter or faster while doing the wrong thing.

Group the conditions without losing precedence

When a large list really is a set of exact, equally ranked hostnames with one outcome, a suitable lookup structure can be clearer than repeated pattern checks. That equivalence must be established first. An exact name, a wildcard and an address-range rule do not automatically belong to one interchangeable set.

Likewise, stripping www. is a policy change if the two names intentionally use different routes. Optimizing a string comparison should not silently redefine the destination.

Remove expensive work the decision does not need

A hostname equality check does not require DNS. Resolve only when the remaining policy needs an address, and avoid repeating the same lookup within one evaluation where the supported PAC environment permits reuse.

Keep a small set of expected decisions beside the file: the specific exception, a sibling under the broad suffix, an unrelated hostname and relevant address literals. Compare the returned route and the order of any fallback entries, not merely whether the JavaScript parses. Once those results match, verify that the browser loaded the revised PAC.

A performance improvement is convincing when the same cases retain their intended routes and unnecessary evaluation work disappears. Broad DIRECT shortcuts can make a delay vanish while changing the exit IP.

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