Open or close an element in fullscreen mode

• Use Document.querySelector() and Element.requestFullscreen() to open the given element in fullscreen.
• Use Document.exitFullscreen() to exit fullscreen mode.
• Omit the second argument, el, to use body as the default element.
• Omit the first element, mode, to open the element in fullscreen mode by default.

const fullscreen = (mode = true, el = 'body') =>
  mode
    ? document.querySelector(el).requestFullscreen()
    : document.exitFullscreen();

// Examples
fullscreen(); // Opens `body` in fullscreen mode
fullscreen(false); // Exits fullscreen mode

Source

https://www.30secondsofcode.org/js/s/fullscreen