how to load Javascript in WordPress Plugin

Solution:

You need to specify when the load should happen, try this.

<?php
/*
Plugin Name: Ava Test
Plugin URI: https://matsio.com
Description: A plugin that is used for my javascript tests
Author: Ronny Kibet
Author URI: https://matsio.com
version: 1.001
*/

add_action('wp_enqueue_scripts','ava_test_init');

function ava_test_init() {
    wp_enqueue_script( 'ava-test-js', plugins_url( '/js/ava_test_.js', __FILE__ ));
}

Also, there are errors in your JS, but I have seen the correct version in some answers, hope this helps

Update : There is a hook called wp_enqueue_scripts, as mentioned by @brasofilo which should be used in lieu of init for loading scripts.