JSFiddle - React, Tailwind, and code Playground

by eligrey

JavaScript

var baseline_measurements = [];
var measurements = 20;
var warmup_runs = 3;

const status = document.documentElement.appendChild(document.createTextNode("DevTools are closed"));
const junk = document.documentElement.insertBefore(document.createElement("div"), document.body);
junk.style.display = "none";
const junk_filler = new Array(1000).join("junk");
const fill_junk = () => {
  var i = 10000;
  while (i--) {
  	junk.appendChild(document.createTextNode(junk_filler));
  }
};
const measure = () => {
	if (measurements) {
    const baseline_start = performance.now();
    fill_junk();
    baseline_measurements.push(performance.now() - baseline_start);
    junk.textContent = "";
    measurements--;
    setTimeout(measure, 0);
  } else {
  	baseline_measurements = baseline_measurements.slice(warmup_runs); // exclude unoptimized runs
    const baseline = baseline_measurements.reduce((sum, el) => sum + el, 0) / baseline_measurements.length;

    setInterval(() => {
      // In actual usage you would also check document.hasFocus()
      // as background tabs are throttled and get false positives
      const start = performance.now();
      fill_junk();
      const time = performance.now() - start;
      status.data = "DevTools are " + (time > 1.77 * baseline ? "open" : "closed");
      junk.textContent = "";
    }, 1000);
  }
};

setTimeout(measure, 200);