SeriveWorker workbox - nodejs and Express

I'm looking for an answer to my problem. I'm testing service worker of workbox with my nodeJs server. I tested with basic configuration like below :

Basic nodeJs app.js

    var http = require('http');
    var fs = require('fs');

    var server = http.createServer(function (req, res) {

        if (req.url === "/") {
            fs.readFile('index.html', function (err, data) {
                if (err) throw err
                res.writeHead(200, {
                    'Content-type': 'text/html;charset=utf-8'
                });
                res.end(data);
            });
        } else if (req.url === '/sw.js') {
            fs.readFile('sw.js', function (err, data) {
                if (err) throw err
                res.writeHead(200, {
                    'Content-type': 'text/javascript'
                });
                res.end(data);
            });
        }
    });

    server.listen(8080);

I managed to put my files into the cache and to make it work in offline mode. But, problems started when I tried to use Express module :

Express nodeJs app.js

    var express = require('express');
    var app = express();

    app.listen(8080);

    //*****************************************************************
    //-------------------------- TEMPLATES --------------------------
    //*****************************************************************
    //moteur de modèles ou de templates
    app.set('view engine', 'ejs');


    //*****************************************************************
    //-------------------------- MIDDLEWARE --------------------------
    //*****************************************************************
    app.use('/static', express.static(__dirname + '/public'));


    //*****************************************************************
    //--------------------------- ROUTES ------------------------------
    //*****************************************************************

    app.get('/', function (request, response) {
        response.render('./pages/index.ejs');
    });

I manage to put my files into the cache but, I don't manage to make it work in offline mode. What is the reason of this difference ?

sw.js

    importScripts('https://storage.googleapis.com/workbox-cdn/releases/3.4.1/workbox-sw.js');

    if (workbox) {
      console.log(`Yay! Workbox is loaded `);
    } else {
      console.log(`Boo! Workbox didn't load `);
    }

      workbox.precaching.precacheAndRoute([
        { url: '/', revision: '383676' }
    ]);

+-node_modules
+-public
  +-js
    sw-register.js
    sw.js
  +-css
+-views
  +-pages
    index.ejs
server.js

EDIT

Don't ask me why, but the problem solved when I added this piece of code into my app.js file, middelware session :

app.use(express.static(__dirname + '/views/pages'));

and inside my "pages" folder I put my sw.js file. In fact, no matter where I put my sw.js file as long as it is inside the folder target by the _dirname.