How to add an ajax/jquery file to a wordpress plugin?

Solution:

Register your script in wordpress like this way http://codex.wordpress.org/Function_Reference/wp_enqueue_script

http://codex.wordpress.org/Function_Reference/wp_register_script

    function my_plugin_admin_init() {
            /* Register our script. */
            wp_register_script( 'my-plugin-script', plugins_url( '/ user_ajax.js', __FILE__ ) );

    wp_enqueue_script('jquery');  // Enqueue jQuery that's already built into WordPress
wp_enqueue_script('my-plugin-script');

}

add_action('wp_enqueue_scripts', 'my_plugin_admin_init');

Use the following function as the way in your wordpress plugin file Use jQuery instead of $.