A more modern approach is WordPress REST using native JavaScript

You should know how WordPress does Ajax.

I recommend that you take a look at the code, it's not hard to understand what's happening.

First I create the data object using FormData.

const data = new FormData();
data.append('action', 'es6_ajax_action');
data.append('nonce', pluginES6WPAjax.nonce);
data.append('sum', self.dataset.sum);

Then I use aync/await with fetch to do the ajax call.

const response = await fetch(url, {
	method: 'POST',
	credentials: 'same-origin',
	body: data
});

const res = await response.json();
if (res.response == 'success') {
	self.dataset.sum = res.data;
	output.innerHTML = res.data;
	console.log(res);
} else {
	console.error(res);
}

Note: previous release use fetch().then().catch()

Note 2: Why move from fetch().then().catch() to async/await? Because V8 ..

favor async functions and await over hand-written promise code

• V8 dev blog

The PHP code is more or less the same as you would do when using jQuery, but I've added the fetch polyfill

// Load the fetch polyfill, url via https://polyfill.io/v3/url-builder/.
wp_enqueue_script( 'polyfill-fetch',
	'https://polyfill.io/v3/polyfill.min.js?features=fetch',
	[],
	ES6_WP_AJAX_DEMO_VERSION,
	true
);

Not very exciting, the demo increments a number when you click on a button.

• Download the plugin
• Install and activate the plugin.
• Add the [es6demo] shortcode to a page.
• Click on the + button to increment the number.

es6-wp-ajax-demo is copyright 2019 Per Soderlind

es6-wp-ajax-demo is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version.

es6-wp-ajax-demo is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with the Extension. If not, see http://www.gnu.org/licenses/.