Node fetch. Take out a JSON

I’m trying to take out the json fetched but It don’t works. Someone can help me?

const fetch = require('node-fetch');
const express = require('express');

const app = express()
const port = 3000

                    .then(res => res.json())
                    .then(json => takeData(json))

let campaniaData;

app.get('/', (req, res) => res.send('It works!'))

app.listen(port, () => console.log(`Example app listening on port ${port}!`))

 function takeData(jsonFile){
    campaniaData = jsonFile