In order to configure Rocket JavaScript please go to Stores > Configuration > Magefan Extensions > Rocket JavaScript.

Rocket JavaScript Settings

1. Choose Yes in the Enabled Deferred JavaScript select box in order to move all of the JavaScripts to the end (bottom) of a page before </html> closing tag.

2. Set the patches of the pages which you don't want to move the JS to the bottom on in the Disallowed Pages for Deferred JavaScript field.

3. Enable the JavaScript Bundling Optimization if you want to manage manually what JS files should be included in the JS bundle.

4. In the Included In Bundling field you can set the JavaScripts you want to be included in the bundle (we recommend this option to be configured by developers).

Magento 2 Rocket JavaScript Configuration

To get a list of used JavaScript files on a storefront page you can use this js script in the browser console:

/* Use in browser console */
globalSrc = '';
if (!jQuery(this).attr('src')) return;
var src = jQuery(this).attr('src');
if (src.indexOf(require.toUrl('')) != -1 && src.indexOf('Magefan_LazyLoad') == -1) {
var src = (src.replace(require.toUrl(''), ''));
globalSrc += "\n" + src;

Once you have finished with the configurations don't forget to press the Save Config button. 

Magento 2 Rocket JavaScript, Save Config

Watch this short video in order to learn how to configure deferred javascript in Magento 2: