Getting img src from thumbnail_id using javascript

I haven’t tested this, but you’ll probably get the gist of it anyway:

add_action('wp_enqueue_scripts', 'send_image_source_to_js');
function send_image_source_to_js() {
    global $post;

    // Does this post have a featured image? If yes, fetch the link / source
    $has_image = $source = false;
    if($thumb_id = get_post_thumbnail_id($post->ID)) {
        $has_image = true;
        $source_array = wp_get_attachment_image_src($thumb_id, array(300,300));
        $source = $source_array[0];
    }

    // Build an array with what we've found about the post
    $image_params = array(
        'has_image' => $has_image ? 1 : 0,
        'source' => $source ? $source : 0
    );

    // Register, localize and enqueue the script
    wp_register_script('my_script', 'path/to/my_script.js');
    wp_localize_script('my_script', 'image_params', $image_params);
    wp_enqueue_style('my_script');
}

What wp_localize_script does is create a Javascript object which will be printed in your page’s html, which you can then use inside your JS code like this (I’m lazy, so I’ve added a non-sensical jQuery example):

$('#my_image').attr('src', image_params.source);

Let us know how it goes!