How to add custom javascript into from custom plugin

You must use the hook to enqueue the scripts in WordPress.
Use the hook wp_enqueue_scriptsfor the front end and admin_enqueue_scripts for the back end side.

If you are enqueueing scripts and styles, you will want to use one of these three hooks:

1. wp_enqueue_scripts (for the frontend)
2. login_enqueue_scripts (for the
   login screen)
3. admin_enqueue_scripts (for the admin dashboard)

It is enough to use one method for register and enqueue of scripts. The difference is the benefit for other developers to de-register the script.

public constructor() {

    add_action( 'wp_enqueue_scripts', array( $this,  'custom_js_register' ) );
}

public function custom_js_register() {

    wp_register_script( 'custom_button', 'http://xxx.xxx.xxx/js/Button.js' );
    wp_enqueue_script( 'custom_button' );

}

Als the hint to not use the static address, with http. It is better to use the wp function plugins_url. Also a example:

    wp_register_script( self::$handle,
           plugins_url( 'js/', __FILE__ ) . 'Button.js'
    );