DataFast shows nothing on localhost? That is on purpose. How to debug it anyway

You added the script, ran next dev, clicked around for ten minutes. DataFast shows zero visitors. The script is fine. It is ignoring you, by design.

What the script skips

• Localhost. localhost, 127.0.0.1, ::1 and *.local hostnames. This keeps your dev clicks out of production numbers.
• Iframes. A page loaded inside an iframe is not tracked unless data-debug="true" is set.
• Your own browser, if you asked. localStorage.datafast_ignore = true in the console excludes that browser on that domain. Easy to forget you did it.
• Bots. Browsers driven by automation (navigator.webdriver, Selenium, PhantomJS) are dropped. Your Playwright suite does not pollute the numbers, and it cannot test tracking either.

Turn it on, briefly

Add data-allow-localhost="true" to the tag. Drive it from an env var so it can never ship to production by accident:

<Script
  src="/js/script.js"
  data-website-id={process.env.NEXT_PUBLIC_DATAFAST_WEBSITE_ID}
  data-domain="example.com"
  data-allow-localhost={
    process.env.NEXT_PUBLIC_DATAFAST_ALLOW_LOCALHOST === "true" ? "true" : undefined
  }
  strategy="afterInteractive"
/>

NEXT_PUBLIC_DATAFAST_ALLOW_LOCALHOST=true

NEXT_PUBLIC_ values are inlined at build time. Restart the dev server after changing it. Set it back to false when you are done. Every event you send from localhost lands in your real dashboard.

data-domain stays your real domain, not localhost.

Walk the request

Open DevTools, Network, and reload.

1. script.js, 200. A 404 means the tag points at a path nothing serves. An empty 200 from your own proxy means it could not reach datafa.st: check the server log.
2. Console. The script logs why it is not tracking: "Tracking disabled on localhost", "inside an iframe", "bot detected". Read that before anything else. The datafast_ignore flag is quieter, so check localStorage too.
3. events, a 2xx. A POST per page view and per goal. None at all means the script decided not to track (step 2). A 4xx is DataFast rejecting the payload: usually a bad website id or a goal name with uppercase letters or spaces.
4. Realtime view. The visit appears within seconds.

Goals that never arrive

• Called before the script loaded, with no queue. Without the queue stub, window.datafast is undefined for the first second of every page view. The call throws or does nothing. Install the stub before anything else runs:

  window.datafast = window.datafast || function (...args) {
    (window.datafast.q = window.datafast.q || []).push(args);
  };
  

• Invalid name. Lowercase letters, digits, _, - and : only, 64 characters max. Signup Completed is rejected.

• More than 10 parameters, or a key with uppercase letters. Rejected or trimmed.

Server goals from localhost

The Goals API does not care where your server runs. It cares about the visitor. It answers 404 when the datafast_visitor_id has no page view on that site. On localhost without data-allow-localhost, no page view was ever recorded, so every server goal 404s. Turn the flag on, load a page, then trigger the action.

Previews

Vercel preview URLs are not localhost, so the script tracks them. They land in production numbers under a *.vercel.app hostname. Exclude that hostname in DataFast's settings, or set the website id only in the production environment.