How to use npm-installed bootstrap in express?

You need to use in your server.js:

app.use(express.static(__dirname + '/node_modules/bootstrap/dist'));

to define a static resourse and then use:

<script language="javascript" src="js/bootstrap.min.js"></script>
<link rel="stylesheet" href="css/bootstrap.min.css"/>

I found a similar question. In which the solution is explained better by @augusto-goncalves. His solution works for me. It is similar to the solution of @victor-behar but more elaborated and clears confusion.