Collect script errors from visitors' browsers

How to do it

Listen for error and unhandledrejection on window. Send the message, the file, the line, the page path and the build with navigator.sendBeacon. Send no query string, form values or user ID. Cap reports at a few per page view, and put the endpoint behind the rate limit. Group reports by message and build, and alert on new ones after a deploy, as in alert on symptoms.

Example

const MAX_REPORTS = 5;
let sent = 0;
function report(message, source = '', line = 0) {
  if (sent++ >= MAX_REPORTS) return;
  navigator.sendBeacon('/errors', JSON.stringify({ message: String(message).slice(0, 500), source, line, path: location.pathname, build: document.documentElement.dataset.build }));
}
addEventListener('error', e => report(e.message, e.filename, e.lineno));
addEventListener('unhandledrejection', e => report(e.reason?.message ?? e.reason));

# Flask: log one line per report, no IP address or cookies
@app.post("/errors")
def browser_errors():
    data = request.get_json(force=True, silent=True) or {}
    app.logger.warning("browser_error build=%s path=%s line=%s message=%r",
        str(data.get("build"))[:40], str(data.get("path"))[:200], data.get("line"), str(data.get("message"))[:500])
    return "", 204

How to check

On a test page, throw an error and reject a promise. Both appear in the server log within a minute, with the build and path and no query string.