Express.js static is serving css files just fine but not javascript files

I’m making a very simple Node application. I’m using CoffeeScript and Pug.

I used this code to set the static files:

app.use express.static('public')

This is my file tree:

This is working fine:

link(rel="stylesheet" href="/css/index.css")

As you can see here

But this is not:

script(src='/js/watch.js')

As you can see here

What have I done wrong?