Example for react-admin to authenticate with OAuth2 against GitLab as the Authorization Server

authProvider.js

import { AUTH_LOGIN } from 'react-admin';

export default (type, params) => {
    if (type === AUTH_LOGIN) {
        const { username, password } = params;
        const oAuthParams = {
            grant_type: "password",
            username,
            password
        }
        const body = Object.keys(oAuthParams).map((key) => {
            return encodeURIComponent(key) + '=' + encodeURIComponent(oAuthParams[key]);
          }).join('&');
        const request = new Request('https://gitlab.com/oauth/token', {
            method: 'POST',
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded;charset=UTF-8'
              },
            body
        })
        return fetch(request)
            .then(response => {
                if (response.status < 200 || response.status >= 300) {
                    throw new Error(response.statusText);
                }
                return response.json();
            })
            .then(( {access_token} ) => {
                localStorage.setItem('token', access_token);
            });
    }
    return Promise.resolve();
}