An ArcGIS JS V4 widget that uses the Fullscreen API.

Include the style, create the widget, then add it to your map UI

require([
  "custom-widgets/FullScreen"
], function(FullScreen) {

  // create your map and view

  var toggle = new FullScreen({
    view: view,
    hideIfFullScreenDisabled: false // default is true
  });
  view.ui.add(toggle, "top-right");

Get it from here or via npm npm install arcgis-fullscreen-widget --save

Download / clone this repository then run npm install > npm start

This uses screenfull internally and was influenced by existing leaflet plugins like https://github.com/brunob/leaflet.fullscreen and https://github.com/Leaflet/Leaflet.fullscreen