read/write JSON file in a simply web application with node.js

I have an HTML file (index.html) where I put a form and I want to see a list of elements that are inside a JSON file (elements.json).
I want to create a server with node.js, so I create app.js in this way:

const express = require('express')
const app = express()
const port = 3000
var config = require('./public/js/elements.json');

//static file
app.use(express.static('public'))
app.use('/css', express.static(__dirname+'public/css'))
app.use('/js', express.static(__dirname + 'public/js'))


//set view
app.set('views', './views')
app.set('view engine', 'ejs')

app.get('', (req, res) => {
    res.sendFile(__dirname+'/views/index.html')
})

//listen
app.listen(port, () => console.info(`Listening on port ${port}`) )

1. How can I do in my script.js file (that is in the same directory of elements.json) to show the elements in a table on my index.html?

2. When I fill the form in the HTML page, I want to add that element on my JSON file, in order to see it in my list of elements on HTML page. How can I do it?