Browser claim probes

The beacon checks whether the browser engine really is the version the User-Agent claims, by probing features that shipped in known versions. A spoofed User-Agent string cannot change what the engine can do.

A scraper can set any User-Agent string it likes, but it cannot retrofit a JavaScript engine. The probe table pairs a browser major version with a function that first shipped natively in it:

var UA_PROBES = [
    63,  'Promise.prototype.finally',
    69,  'Array.prototype.flat',
    73,  'Object.fromEntries',
    85,  'String.prototype.replaceAll',
    93,  'Object.hasOwn',
    98,  'structuredClone',
    110, 'Array.prototype.toSorted',
    122, 'Set.prototype.union'
];

Given a User-Agent claiming version N: every entry at or below N minus two must be present, or the engine is older than claimed; every entry at or above N plus two must be absent, or the engine is newer than claimed. The grace of two majors means a browser mid-upgrade, an enterprise pin, or one of the browser’s own User-Agent reduction quirks is never flagged.

Adding a row as the browser advances

Roughly every ten releases, pick a feature that is shipped in a known version — look it up rather than guessing, because a wrong version number here manufactures false accusations against real people — that is reachable as a function by a dotted path, and that is not commonly polyfilled. Never remove old rows; they are what catches ancient engines.