i'm using a basic forum using node.js and express writted there, and i want to switch to mongodb but i'm very new to it. I've already tried a few steps but it's not going anywhere. I just want to implement my database following the same way as this code.

var router  = express.Router();
var fs      = require('fs');

router.get('/forum',(req,res)=>{
  res.render('forum',{db:req.app.locals.frm,'user':req.session.user});
})
.post('/forum',(req,res)=>{
  var page = req.body;
  req.app.locals.frm.push({'category':page.cat,'subcategories':[]});
  fs.writeFile('./forum-db.json',JSON.stringify(req.app.locals.frm),(e)=>{console.log(e);});
  res.redirect('/forum');
})
.get('/del-elt/:pos',(req,res)=>{
  var page = req.params;
  req.app.locals.frm.splice(page.pos,1);
  fs.writeFile('./forum-db.json',JSON.stringify(req.app.locals.frm),(e)=>{console.log(e);});
  res.redirect('/forum');
})
.get('/category/:id',(req,res)=>{
  var cat = req.app.locals.frm[req.params.id]
  res.render('category',{cat:cat,id:req.params.id,'user':req.session.user})
})
.post('/category',(req,res)=>{
  var page = req.body;
  req.app.locals.frm[page.id].subcategories.push({'title':page.cat,'discussions':[]});
  fs.writeFile('./forum-db.json',JSON.stringify(req.app.locals.frm),(e)=>{console.log(e);});
  res.redirect('/category/'+page.id);
})
.get('/del-cat/:id/:pos',(req,res)=>{
  var page = req.params;
  req.app.locals.frm[page.id].subcategories.splice(page.pos,1);
  fs.writeFile('./forum-db.json',JSON.stringify(req.app.locals.frm),(e)=>{console.log(e);});
  res.redirect('/category/'+page.id);
})
.get('/discussion/:cat/:id',(req,res)=>{
  var dis = req.app.locals.frm[req.params.cat].subcategories[req.params.id];
  res.render('discussion',{dis:dis,cat:req.params.cat,id:req.params.id,'user':req.session.user})
})
.post('/discussion',(req,res)=>{
  var page = req.body;
  var user = req.session.user;
  var d = new Date();
  var date = d.getDate()+"/"+d.getMonth()+"/"+d.getFullYear()+" ("+d.getHours()+":"+d.getMinutes()+")";
  req.app.locals.frm[page.cat].subcategories[page.id].discussions.push([ page.message,user.mail,date ]); //+mail+date
  fs.writeFile('./forum-db.json',JSON.stringify(req.app.locals.frm),(e)=>{console.log(e);});
  res.redirect('/discussion/'+page.cat+'/'+page.id);
})
.get('/del-dis/:cat/:id/:pos',(req,res)=>{
  var page = req.params;
  req.app.locals.frm[page.cat].subcategories[page.id].discussions.splice(page.pos,1);
  fs.writeFile('./forum-db.json',JSON.stringify(req.app.locals.frm),(e)=>{console.log(e);});
  res.redirect('/discussion/'+page.cat+'/'+page.id);
});

If anyone has any clue on how to do it. Thank you