video.js/src/js/control-bar/fullscreen-toggle.js

/**
 * Toggle fullscreen video
 * @param {vjs.Player|Object} player
 * @param {Object=} options
 * @class
 * @extends vjs.Button
 */
vjs.FullscreenToggle = vjs.Button.extend({
  /**
   * @constructor
   * @memberof vjs.FullscreenToggle
   * @instance
   */
  init: function(player, options){
    vjs.Button.call(this, player, options);
  }
});

vjs.FullscreenToggle.prototype.buttonText = 'Fullscreen';

vjs.FullscreenToggle.prototype.buildCSSClass = function(){
  return 'vjs-fullscreen-control ' + vjs.Button.prototype.buildCSSClass.call(this);
};

vjs.FullscreenToggle.prototype.onClick = function(){
  if (!this.player_.isFullScreen()) {
    this.player_.requestFullScreen();
    this.el_.children[0].children[0].innerHTML = 'Non-Fullscreen'; // change the button text to "Non-Fullscreen"
  } else {
    this.player_.cancelFullScreen();
    this.el_.children[0].children[0].innerHTML = 'Fullscreen'; // change the button to "Fullscreen"
  }
};