screencap.js

async function screenCap(duration) {
	function _startScreenCapture() {
	    if (navigator.getDisplayMedia) {
	      return navigator.getDisplayMedia({video: true});
	    } else {
	      return navigator.mediaDevices.getUserMedia({video: {mediaSource: 'screen'}});
	    }
	}

	const stream = await _startScreenCapture();

	const downloadLink = document.createElement('a');
	downloadLink.textContent = 'Screen Recording';
	downloadLink.style = 'position: absolute; bottom: 0; left: 0;';
	document.body.appendChild(downloadLink);

	const videoChunks = [];
    const mediaRecorder = new MediaRecorder(stream, {mimeType: 'video/webm'});
    mediaRecorder.addEventListener('dataavailable', event => {
      if (event.data && event.data.size > 0) {
        videoChunks.push(event.data);
      }
    });
    mediaRecorder.start(10);

    const stopRecording = () => {
    	mediaRecorder.stop();
    	stream.getTracks().forEach(track => track.stop());

    	const recordingUrl = window.URL.createObjectURL(new Blob(videoChunks, {type: 'video/webm'}));

	    downloadLink.download = 'screen-recording.webm';
	    downloadLink.href = recordingUrl;
    };

    setTimeout(stopRecording, duration);
}