server.js

const DATASOURCE = 'data.json';

const fs = require('fs');
const parser = require('body-parser');
const app = require('express')();

app.use(parser.json());

// HTTP GET /api/data
app.get('/api/data', (req, res, next) => {
    fs.readFile(DATASOURCE, (err, data) => {
        if(err) { return next(err); }

        res.json(data);
    });
});

// HTTP POST /api/data { content }
app.post('/api/data', (req, res, next) => {
    fs.writeFile(DATASOURCE, req.body.content, (err) = {
        res.json({
            error: err || null
        });
    });
});

app.listen(8080, () => {
    console.log('Server up and running ...');
});