How to deal with placeholder in URL?

dogs

Hey! I set up a litte plugin.

plugin.json

"hooks": [
   {
      "hook": "static:app.load", "method": "init"
   }
]

libary.js - plugin.init()

router.get('/projects', hostMiddleware.buildHeader, controllers.getCustomParam);
router.get('/projects/:tag', hostMiddleware.buildHeader, controllers.getCustomParam);

controllers.js - Controllers.getCustomParam()

// Template which will be rendered
res.render('admin/plugins/mytemplate', {
   title: "myTitle",
   sub_title: "thisIsASub"
});

Everything works fine so far. But how can I deal with placeholder /projects/:tag e.g

/projects/diy
/projects/mechanics
/projects/electric

How can I get the value of each :tag and in which file do I have declare my functions?
How can I pass data from libary.js into my controllers.js?

I want to do some code and functions and pass this data into my template to display it to the user.

It would be nice if anyone could help me.

<baris>

req.params.tag will give you the parameter.