A page to trigger AJAX and Display the Output.

/**
Plugin Name:  ajax test
Plugin URI: https://deshisoft.com/plugins/ajax/
Description: Ajax Test shortcode [aj_exam].
Author: Tamal Sarker
Version: 1.0.0
Author URI: http://deshisoft.com/
*/

/* Set constant path to the plugin directory. */
define('EYE_PATH', plugin_dir_path(__FILE__));
/* Set the constant path to the plugin directory URI. */
define('EYE_URL', plugin_dir_url(__FILE__));

/* Add CSS */
function eye_styles(){
	wp_register_style( 'header_cart_css', EYE_URL . 'assets/css/header-cart.css' , array(), '1.1.0', false );
	wp_enqueue_style( 'header_cart_css' );
}
add_action('wp_head', 'eye_styles', 5);

/* Add JS */
function eye_scripts() {
	wp_register_script( 'header_cart_js', EYE_URL . 'assets/js/header-cart.js', array('jquery'), '1.0.0', true );
	wp_enqueue_script( 'header_cart_js' );
	wp_localize_script('header_cart_js', 'ajax_custom', array(
		'ajaxurl' => admin_url('admin-ajax.php'),
		'nonce' => wp_create_nonce('ajax-nonce'),
		'getID' => get_the_ID()
	));
}
add_action('wp_footer', 'eye_scripts', 5);

//call post function
require_once(EYE_PATH . 'function.php');

add_shortcode('aj_exam', 'aj_exam');
function aj_exam(){
	$out = '<div id="show-enquiry"></div>'; // Click Output
	$out .= '<button id="add-enquiry">Add Enquiry</button>'; // Click Input
	return $out;
}

function.php

function enquiry_ajax_request() {

	$args = array(  
		'post_type' => 'post',
		'post_status' => 'publish',
		'posts_per_page' => 5, 
	);

	$loop = new WP_Query( $args ); 

	while ( $loop->have_posts() ) : $loop->the_post(); ?>
		<h3><?php the_title(); ?></h3>
        <p><?php the_content(); ?></p>
        <?php the_post_thumbnail(); ?>
	<?php endwhile;
	
	wp_die();
	
}
add_action( 'wp_ajax_enquiry_ajax_request', 'enquiry_ajax_request' );
add_action( 'wp_ajax_nopriv_enquiry_ajax_request', 'enquiry_ajax_request' );

header-cart.js

jQuery(document).ready(function($) {
    jQuery( "#add-enquiry" ).on('click',function() {      
        $.ajax({
            url: ajax_custom.ajaxurl,
            data: {
                'action':'enquiry_ajax_request'
				//'post_id' : header_cart_js.getID
            },
            success:function(data) {
                //alert('okay');
				//console.log(data);
				$('#show-enquiry').html(data);
            },
            error: function(errorThrown){
                console.log(errorThrown);
            }
        });
    });
});