<video id="videoElement" controls="true" src="demo.mp4"></video>

<!-- button will be used to toggle the PiP mode -->
<button id="togglePipButton">Toggle Picture-in-Picture Mode!</button> 

let video = document.getElementById('videoElement');
let togglePipButton = document.getElementById('togglePipButton');

togglePipButton.addEventListener('click', async function (event) {
    togglePipButton.disabled = true; //disable toggle button while the event occurs
    try {
        // If there is no element in Picture-in-Picture yet, request for it
        if (video !== document.pictureInPictureElement) {
            await video.requestPictureInPicture();
        }
        // If Picture-in-Picture already exists, exit the mode
        else {
            await document.exitPictureInPicture();
        }

    } catch (error) {
        console.log(`Oh Horror! ${error}`);
    } finally {
        togglePipButton.disabled = false; //enable toggle button after the event
    }
});

let video = document.getElementById('videoElement');
video.addEventListener('enterpictureinpicture', function (event) {
    console.log('Entered PiP');
    pipWindow = event.pictureInPictureWindow;
    console.log(`Window size -  \n Width: ${pipWindow.width} \n Height: ${pipWindow.height}`); // get the width and height of PiP window
});

video.addEventListener('leavepictureinpicture', function (event) {
    console.log('Left PiP');
    togglePipButton.disabled = false;
});

if ('pictureInPictureEnabled' in document) {
    showPipButton();

    // loadedmetadata event occurs when meta data for the specified audio/video has been loaded.Meta data might consists of: duration, dimensions etc.
    video.addEventListener('loadedmetadata', showPipButton);

    // emptied event is fired when the media has become empty, e.g. media has already been loaded or partially loaded.
    video.addEventListener('emptied', showPipButton);
} else {
    console.log('The Picture-in-Picture Web API is not available.');
    togglePipButton.hidden = true;
}

// Enable/disable toggle button depending on whether PiP availability.
function showPipButton() {
    togglePipButton.disabled = (video.readyState === 0) || !document.pictureInPictureEnabled || video.disablePictureInPicture;
}
// video.readyState === 0 means Video metadata have not been loaded yet 
// !document.pictureInPictureEnabled means if Pip is not available
// video.disablePictureInPicture means disablePictureInPicture attribute is present in the video which will result in requestPictureInPicture() rejected promise.