Measure Time to first byte

httpPing.js

// time to first byte
const https = require('https');
const http = require('http');

/**
 * @param {Object} arg
 * @param {String} arg.host
 * @param {String} arg.path
 * @param {{ [k: string]: string }} arg.headers
 */
module.exports = function httpPing({
  isHttps = true,
  host,
  path,
  headers,
}) {
  return new Promise((resolve) => {
    const start = Date.now();
    const req = (isHttps ? https : http).request({
      host,
      path,
      headers,
    }, (res) => {
      let body = '';

      res.once('data', () => {
        if (res.statusCode <= 304) {
          resolve([Date.now() - start, res.statusCode]);
          req.destroy();
        }
      });

      res.on('data', (d) => {
        body += d;
      });

      res.on('end', () => {
        resolve([Date.now() - start, res.statusCode, body]);
      });
    });

    req.on('error', (error) => {
      resolve([Date.now() - start, -1, error]);
    });

    req.end();
  });
};