philipboomy/axios.js

## axios.js
import qs from 'qs';
import Axios from 'axios';

Axios.defaults.baseURL = Craft.baseUrl;
Axios.defaults.headers.common['Accept'] = 'application/json';
Axios.defaults.headers.common['X-Requested-With'] = 'XMLHttpRequest';
Axios.defaults.headers.post['Content-Type'] = 'application/x-www-form-urlencoded';

Axios.interceptors.request.use(config => {
    if (config.data instanceof FormData) {
        config.data.append(Craft.csrfTokenName, Craft.csrfToken);
    } else if (config.data === Object(config.data)) {
        config.data = qs.stringify(Object.assign({[Craft.csrfTokenName]: Craft.csrfToken}, config.data));
    }

    return config;
});

window.axios = Axios;

## base.twig
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <meta http-equiv="x-ua-compatible" content="ie=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

    <title>Axios with Craft</title>

    <link rel="stylesheet" href="{{ alias('@web/assets/app.css') }}">

    <script src="{{ alias('@web/assets/vendor.js') }}" defer></script>
    <script src="{{ alias('@web/assets/app.js') }}" defer></script>

    {% set jsCraft = {
        baseUrl: alias('@web'),
        csrfTokenName: craft.app.config.general.csrfTokenName,
        csrfToken: craft.app.request.csrfToken,
    } %}

    <script>window.Craft = {{ jsCraft|json_encode|raw }}</script>
</head>

<body>
    <div id="app">
        {% block layout %}
        {% endblock %}
    </div>
</body>
</html>
	import qs from 'qs';
	import Axios from 'axios';

	Axios.defaults.baseURL = Craft.baseUrl;
	Axios.defaults.headers.common['Accept'] = 'application/json';
	Axios.defaults.headers.common['X-Requested-With'] = 'XMLHttpRequest';
	Axios.defaults.headers.post['Content-Type'] = 'application/x-www-form-urlencoded';

	Axios.interceptors.request.use(config => {
	if (config.data instanceof FormData) {
	config.data.append(Craft.csrfTokenName, Craft.csrfToken);
	} else if (config.data === Object(config.data)) {
	config.data = qs.stringify(Object.assign({[Craft.csrfTokenName]: Craft.csrfToken}, config.data));
	}

	return config;
	});

	window.axios = Axios;
	<!DOCTYPE html>
	<html lang="en">
	<head>
	<meta charset="utf-8">
	<meta http-equiv="x-ua-compatible" content="ie=edge">
	<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

	<title>Axios with Craft</title>

	<link rel="stylesheet" href="{{ alias('@web/assets/app.css') }}">

	<script src="{{ alias('@web/assets/vendor.js') }}" defer></script>
	<script src="{{ alias('@web/assets/app.js') }}" defer></script>

	{% set jsCraft = {
	baseUrl: alias('@web'),
	csrfTokenName: craft.app.config.general.csrfTokenName,
	csrfToken: craft.app.request.csrfToken,
	} %}

	<script>window.Craft = {{ jsCraft\|json_encode\|raw }}</script>
	</head>

	<body>
	<div id="app">
	{% block layout %}
	{% endblock %}
	</div>
	</body>
	</html>