How to render a pug view without removing existing data?

For example, I have a pug view 'VIEW.pug', it contains two parts

• part A: with some data

• part B: empty

after request a GET from express/node, I would like to keep part A's data and refresh part B with new data by using the res.render('VIEW', {xxx}) function.

So how can I make this happen? Once I render VIEW.pug, though I refresh part B with the correct data, but the data in part A are gone.

I wanna both of part A and part B have data, A with old existing data, and B with new data.

create a layout.pug file :

html
  head
    title My Site - #{title}
    block scripts
      script(src='/jquery.js')
  body
    block content

now create a file with partA.pug , which extends layout file :

-extends layout.pug
  block content
  h1= PART A

now use include to include partB in partA file

 include includes/partB.pug

your partB.pug should be rendered based on condition

#partB
- if(section=== 'DEMO-1') {
| <p>DEMO 1</p>
- }
- if(section === 'DEMO-2') {
| <p>DEMO 2</p>
- }

app.js

var express=require('express');
let app = express();
app.set("view engine", "pug");
app.set("views", path.join(__dirname, "views"));


app.get('/demo1',function(req,res){
   return res.render("partA",{section:'DEMO-1'});
});

app.get('/demo2',function(req,res){
   return res.render("partA",{section:'DEMO-2'});
});