Defer-Javascript WordPress

defer-javascript.php

<?php 
declare( encoding = 'UTF-8' );
// Optional Junk
/**
 * Plugin Name: Defer Scripts
 * Description: Add defer to enqueued javascripts
 * Version:     1.0
 * Required:    3.3
 * Author URI:  http://www.gnu.org/
 * License:     GNU GPL v3.0
 * License URI: http://www.gnu.org/copyleft/gpl.html
 *
 */
// You can modify and use your own name above

! defined( 'ABSPATH' ) and exit;

// Change YOURID to small letter specific name
 
if ( ! function_exists( 'add_defer_to_YOURID' ) )
{
	function add_defer_to_YOURID( $url )
	{
	    if ( 
// We are deferring a specific script, the plugin is plugin-name
	    FALSE === strpos( $url, 'plugin-name' ) or
	    FALSE === strpos( $url, '.js' )
	    )
	    { 
	        return $url;
	    }
// Get the things done
	    return "$url' defer='defer";
	}
	// The number should not conflict with any eneque function 
	add_filter( 'clean_url', 'add_defer_to_YOURID', 15, 7 );
}

Read here : http://thecustomizewindows.com/2014/04/defer-parsing-javascript-wordpress-snippets-plugins/
And here : http://thecustomizewindows.com/2013/11/eliminate-render-blocking-javascript-and-css-wordpress/
For more details.