In javascript, do something if the featured image (post thumbnail) is updated

admin.js

// Do something if the featured image is updated by inspecting ajaxComplete callback

jQuery(document).ajaxComplete(function(event, xhr, settings){
  // Parse the request
	var vars = {};
	var requestVars = settings.data.replace(/[?&]+([^=&]+)=([^&]*)/gi, function (m, key, value) {
		vars[key] = value;
	});

	// Take action if the action parameter is for the thumbnail
	if ( vars.action === 'set-post-thumbnail') {
		// Do something.
	}

});