express-middleware-server-timing/index.js

Generate a server-timing header for every request.

const onHeaders = require("on-headers");
const headerName = "Server-Timing";

module.exports = function (app, options = {}) {
  const name = options.name || "mw";
  const description = options.description;
  const listen = app.listen;
  const disabled = app.disabled === true;

  function appendHeader(res, startTime, name, description) {
    if (disabled) {
      return;
    }
    const diff = process.hrtime(startTime);
    const headerString = []
      .concat(res.getHeader(headerName) || [])
      .concat(toServerTimingEntry(name, diff, description))
      .join(",");
    res.setHeader(headerName, headerString);
  }

  app.listen = function () {
    // add our middleware to the end
    app.use(function (req, res, next) {
      const startTime = process.hrtime();
      onHeaders(res, function () {
        appendHeader(res, startTime, name, description);
      });

      const timers = {};
      res.serverTimingStart = function (name, description) {
        const startTime = process.hrtime();
        timers[name] = {
          startTime,
          description,
        };
      };

      res.serverTimingStop = function (name) {
        const timer = timers[name];
        if (!timer) {
          console.warn(`no timer named ${name}`);
          return;
        }

        appendHeader(this, timer.startTime, name, timer.description);
        delete timers[name];
      };

      res.serverTimingSync = function (method, name, description) {
        const startTime = process.hrtime();
        const returnValue = method();
        appendHeader(this, startTime, name, description);
        return returnValue;
      };

      next();
    });

    // move the Layer we just added from last to first
    // yes, this is probably a bad idea that won't work forever, but...
    app._router.stack.unshift(app._router.stack.pop());

    // let the listening begin
    return listen.apply(app, arguments);
  };
};

function toServerTimingEntry(name, diff, description) {
  const entry = [];
  entry.push(name);
  entry.push(`dur=${diff[0] * 1e3 + diff[1] / 1e6}`);
  if (description) {
    entry.push(`desc=${JSON.stringify(description)}`);
  }
  return entry.join("; ");
}