const http = require('http'),
      express = require('express'),
      app = express();

app.use(express.static(__dirname + '/front-end'));
app.use('/add', express.static(__dirname + '/front-end/add.html'));
app.listen(8080);

<input type='button' name='button' value='ОК' id='button'>

<input type='submit' name='button' value='ОК' id='button'>