Trouble with the "express" module with Node JS

Hello.

I am learning Node JS with Visual Studio Code from Microsoft. Here is the Javascript file:

«test3.js»:

const express = require('express')
const app = express()
const port = 9999

app.get('/', function (request, response) {
  response.send('Allo de Express!')
})

app.listen(port, function (err) {
  if (err) {
    return console.log('Oups! Une erreur est survenue', err)
  }

  console.log(`À l'écoute sur le port ${port}`)
})

I had installed «express» with this command:

PS C:\Users\jean\Documents> npm -g install express
+ express@4.16.3
added 1 package, updated 2 packages and moved 2 packages in 11.24s

PS C:\Users\jean\Documents> cd node_modules
PS C:\Users\jean\Documents\node_modules> ls


    Répertoire : C:\Users\jean\Documents\node_modules


Mode                LastWriteTime         Length Name
----                -------------         ------ ----
d-----       29/03/2018     14:59                bytes

There no directory for the module «express». Where is it?

When I execute the Javascript file, I get an error:

PS C:\Users\jean\Documents> node test3.js
module.js:549
    throw err;
    ^
Error: Cannot find module 'express'
    at Function.Module._resolveFilename (module.js:547:15)
    at Function.Module._load (module.js:474:25)
    at Module.require (module.js:596:17)
    at require (internal/module.js:11:18)
    at Object.<anonymous> (C:\Users\jean\Documents\test3.js:1:79)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)

But the module «express» seems to be there:

PS C:\Users\jean\Documents> npm search express

NAME                      | DESCRIPTION          | AUTHOR          | DATE       | VERSION  | KEYWORDS
express                   | Fast,…               | =dougwilson…    | 2018-03-12 | 4.16.3   | express

Any help, please.

Jean