node js store returned API key as variable

I am new to node and am trying to using A node.js library for the Pardot API

This I pass my userKey, email and password to pardot, this returns my api_key.

I am using the following code to authenticate, then when I run my node server I can see the api_key being passed back.

how do I then store that api_key as a variable to then use to post data to the API?

my node code

var nodePardot = require('node-pardot');

// Create client using email, password and user_key
nodePardot.PardotAPI({
    userKey: 'kyujk',
    email: 'uu.uuy@uu.co.uk',
    password: 'uit.09',
    DEBUG: true
}, function(err, client) {
    // Authentication completed
    console.log('Authentication completed');

    // gets api key
    var api_key = api_key;
    console.log('key',api_key);
});

my console output

[nodemon] restarting due to changes...
[nodemon] starting `node index.js`
Server started! At http://localhost:8080
Initializing node-pardot in DEBUG mode
params {"email":"bhyuyten.jj@jyj.co.uk","password":"tyjt.09","user_key":"yjty76","format":"json"}
uri https://pi.pardot.com/api/login/version/3?email=ss.jyj%s.co.uk&password=tyjt.09&user_key=yjty76&api_key=&format=json
parsedResponse {"@attributes":{"stat":"ok","version":1},"api_key":"878074492492dhjjk67u","version":4}
Authentication completed
key undefined
parsedResponse {"@attributes":{"stat":"ok","version":1},"api_key":"878074492492dhjjk67u","version":4}


var nodePardot = require('node-pardot');

    // Create client using email, password and user_key
    nodePardot.PardotAPI({
        userKey: 'fdgr',
        email: 'rg.gg@gg.co.uk',
        password: 'dfherg',
        DEBUG: true
    }, function(err, client) {
        // Authentication completed
        console.log('Authentication completed');

        // gets api key
        var api_key = api_key;
        console.log('key',api_key);
    });

via Beep