I built 118+ device tests that run entirely in your browser — no server, no uploads

Every time I needed to check whether my microphone actually worked before a call, or whether a key on a used keyboard was dead, I ended up on some sketchy "free online tester" that wanted an account, showed six ad banners, and — the worst part — happily streamed my webcam to a server I knew nothing about. So I built the thing I actually wanted: TestOnDevice — 118+ device tests that run 100% in the browser. No installs, no account, and nothing you record ever leaves your machine. Here's what I learned building it. The core constraint: the network is off-limits The single rule that shaped the whole project: no device data touches a server. No webcam frames, no audio buffers, no keystrokes, no sensor readings. That constraint turned out to be freeing rather than limiting, because modern browser APIs are genuinely powerful. Almost every "device test" is just a Web API plus a bit of rendering: What you're testing The API doing the work Microphone / speakers MediaDevices.getUserMedia , Web Audio ( AnalyserNode ) Webcam getUserMedia , <video> , MediaStreamTrack.getSettings() Keyboard keydown / keyup , KeyboardEvent.code Mouse / touch / stylus Pointer Events, PointerEvent.pressure Gamepads Gamepad API MIDI keyboards Web MIDI API Display ( dead pixels , refresh rate ) Fullscreen + requestAnimationFrame Motion / orientation DeviceOrientationEvent , DeviceMotionEvent A live mic meter, entirely local The microphone test is a good example of how little you need. Grab a stream, wire it into an AnalyserNode , and compute RMS on each frame for a level meter — no recording, no upload: const stream = await navigator . mediaDevices . getUserMedia ({ audio : true }); const ctx = new AudioContext (); const analyser = ctx . createAnalyser (); ctx . createMediaStreamSource ( stream ). connect ( analyser ); const data = new Uint8Array ( analyser . frequencyBinCount ); function tick () { analyser . getByteTimeDomainData ( data ); let sum = 0 ; for ( const v of data ) { const x = ( v - 128 )