how to use jQuery installed with npm in Express app?

If you want a jquery npm module to be served by an express app then add this line to the server script (in your case app.js):

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

After that you can include it in your html file:

<script src="/jquery/jquery.js"></script>

When you are installing jQuery with npm it's because you want to use jQuery on the server side of your application (Ex : in your app.js file). You still need to add jQuery to your web page like that :

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>

If you want to use it on the client side. If you are using Jade, add the script tag to your template.