<?php 
add_shortcode('aj_likes_btn', 'aj_likes_btn');
function aj_likes_btn(){
	global $post;

	$post_id = $post->ID;
	$current_count = get_post_meta( $post_id, 'likes',true);
	$current_count = $current_count ? $current_count : 0;
    
    ob_start();
	?>
	<div class='aj_likes'>
		<p><a class='aj_thumbsup' href='' data-id='<?php echo $post_id;?>'>
		    <i class="far fa-thumbs-up"></i></a> 
		    Likes (<span><?php echo $current_count;?></span>)</p>
	</div>
	<?php
	return ob_get_clean();
}

jQuery(document).ready(function($){

$('body').on('click','.aj_thumbsup',function(event){
	event.preventDefault(); // stop click from refreshing page
    
    var OBJ = $(this);
    
    if(OBJ.hasClass('recorded')) return; // stop multiple submissions
    
	var id = $(this).data('id'); // get post ID
	var data_arg = {};
	data_arg['id'] = id;
	data_arg['action'] = 'aj_like_post'; // ajax hook name

	$.ajax({
		beforeSend: function(){},
		type: 'POST',
		url:the_ajax_script.ajaxurl, // Must be updated if adding this to existing
		data: data_arg,dataType:'json',
		success:function(data){
			OBJ.siblings('span').html( data.new_count); // populate with new new_count
			OBJ.addClass('recorded') // mark as thumbsup recorded
			OBJ.find('i').removeClass('far').addClass('fas'); // make thumbs up icon solid
		}
	});
});
});

<?php
define("THEME_DIR", get_template_directory_uri());
wp_enqueue_script( 'aj_script', THEME_DIR . '/aj_script.js', array( 'jquery' ), 1.0, true );
wp_localize_script( 
	'aj_script', // the handle name of the script
	'the_ajax_script', 
	apply_filters('aj_script_data', array( 
		'ajaxurl' => admin_url( 'admin-ajax.php' ) , 
		'postnonce' => wp_create_nonce( 'aj_nonce' )
	))
);

<?php
add_action( 'wp_ajax_aj_like_post', 'aj_record_like_post' );
add_action( 'wp_ajax_nopriv_aj_like_post', 'aj_record_like_post' ); 
// nopriv allows for nonloggedin users to like as well

public function aj_record_like_post(){

	$post_id = sanitize_text_field( $_POST['id']);

	$current_count = get_post_meta( $post_id, 'likes',true);
	$current_count = $current_count ? $current_count : 0;

	$new_count = $current_count + 1; // add one more like

	update_post_meta( $post_id, 'likes', $new_count); // save new count to the post meta

	echo json_encode(array(
		'status'=>'good',
		'new_count'=>$new_count,
	)); exit;
}

.aj_likes{text-align:center; padding:30px 0; font-size:18px;}
.aj_likes a:hover i{transform: scale(1.2);}
.aj_likes a.recorded:hover i{transform: scale(1);}