How to interact MongoDB/Express Router files with clientside javascript files

I had a relatively old clientside javascript/html/css project that I want to hook up to a MongoDB server. I already set up the express folder so that the clientside is running correctly. However, I have no idea how to access the DB from clientside(which is where I'll need to extract the data initially). I also don't know how to connect the nodejs files(like from /router and app.js) to the javascript files in the clientside. It feels to me that the clientside is separated from MongoDB/nodejs and I'm sure there's a way to do it - I've searched but there were no useful results. Any idea on how to do this?

Ex.

router.get('/main.html', function(req, res) {
res.render('main', { title: 'Express' });
var db = req.db;

var username = localStorage.getItem('company');//contains the info to check if there is an overlap.
var password = localStorage.getItem('key');

var collection = db.get('usercollection');//gets the specific collection group
//if(wantUpdate){
  collection.remove( [{"username" : username} , {"password" : password}] );
//}
//else{

//}

if(collection.find( [{"username" : username} , {"password" : password}]).count() == 0)//means there is no need to add an entry
{
  collection.insert( [{"username": username} , {"password" : password}, {"jsonMain" : /*HOW DO I FIND THIS*/}, {"jsonMilestones" : /*HOW DO I FIND THIS*/}, {"jsonTasks" : /*HOW DO I FIND THIS*/}]);
}
});

note: Not good with this code block thing - I swear I indent properly in my programs!