File starts with a UTF-8 byte order mark
medium
The first three bytes are a UTF-8 BOM, typically written by Windows editors. Modern browsers strip it; older or embedded engines may fail to parse the file.
Why it matters
A BOM is not part of JavaScript syntax. V8 and SpiderMonkey skip it, so Chromium and Firefox are unaffected. Reports exist of WinHTTP-based clients and embedded engines treating it as an invalid token, which turns the whole file into a parse error (silent DIRECT). It is also a sign that the file was edited with a tool that may have introduced other non-ASCII characters (PAC-E014, PAC-E015). A PAC file should be plain 7-bit ASCII.
Draft: the WinHTTP behaviour is from field reports and needs a lab run.
How to fix
Save the file as UTF-8 without BOM (or plain ASCII); check with `hexdump -C file | head -1`.
Examples
Bad
// (the file begins with the invisible bytes EF BB BF before this comment)
function FindProxyForURL(url, host) {
return "DIRECT";
}
Engine behaviour
| Engine | Behaviour | Source |
|---|---|---|
| chromium | BOM is skipped by the parser. | expert, unverified |
| winhttp | Reported to fail parsing; needs lab confirmation. | expert, unverified |