FindProxyForURL Beta

PAC-B009 · mutable-result-variable

Result assembled in a variable instead of returned per rule

low · Best practice and maintainability

var proxy = ...; if (a) proxy = ...; if (b) proxy = ...; return proxy; inverts the usual semantics (last match wins instead of first) and evaluates every condition, including DNS lookups, for every request.

Why it matters

PAC authors and reviewers expect first-match-wins, which a sequence of if (...) return gives for free. The accumulator style evaluates all branches (so every expensive predicate runs), lets a later broad rule override an earlier specific one, and spreads the decision over the whole function. It is also where missing default assignments hide.

How to fix

Return directly from each branch and end with an unconditional default return.

Examples

Bad

function FindProxyForURL(url, host) {
  var result = "PROXY proxy.corp.example:8080";
  if (isPlainHostName(host)) { result = "DIRECT"; }
  if (dnsDomainIs(host, ".corp.example")) { result = "DIRECT"; }
  if (shExpMatch(host, "*.priority.example")) { result = "PROXY fast.corp.example:8080"; }
  return result;
}
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Good

function FindProxyForURL(url, host) {
  if (isPlainHostName(host)) { return "DIRECT"; }
  if (dnsDomainIs(host, ".corp.example")) { return "DIRECT"; }
  if (shExpMatch(host, "*.priority.example")) { return "PROXY fast.corp.example:8080"; }
  return "PROXY proxy.corp.example:8080";
}
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