function getFruits() {
    return json_encode( ['fruits' => ['apples', 'pears']]);
}

function my_enqueue() {
    wp_enqueue_script( 'ajax-script', get_template_directory_uri() . '/js/my-ajax-script.js', array('jquery') );

    wp_localize_script( 'ajax-script', 'my_ajax_object',
        array( 'ajax_url' => admin_url( 'admin-ajax.php' ) ) );
}
add_action( 'wp_enqueue_scripts', 'my_enqueue' );

$(document).ready(function() {
    $.ajax(
        {
            type: "get",
            data : {
                action: 'getFruits'
            },
            dataType: "json",
            url: my_ajax_object.ajax_url,
            success: function(msg){
                console.log(msg); //output fruits to console
            }
        });
});