...
"scripts": {
   "build": "set NODE_ENV=production && webpack --config ./webpack.prod.config.js --progress --colors",
   "start": "node ./app.js"
},
...

let userpass = btoa(`${clientId}:${clientSecret}`);

let url_2 = `https://api.github.com/applications/${clientId}/tokens/${x.access_token}`;
fetch(url_2, { 
            method: 'GET',
            headers: {
                'Authorization': `Basic ${userpass}`
            }
        } )
.then(x => x.json())
.then(x => {console.log(x)});