Send response in chunks

chunck-response.js

const express = require('express');
const app = express();

const wait = (sec) => new Promise((res, rej) => setTimeout(res, sec*1000));

app.get('/yes', (req, res) => {
  // 8 sec for header
  wait(8)
  // 8 * 2 = 16 sec for body
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.end('Bye World!');
    });
});

app.get('/to-header', (req, res) => {
  // 11 sec for header
  wait(11)
  // 8 * 2 = 16 sec for body
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.end('Bye World!');
    });
});

app.get('/to-body', (req, res) => {
  // 8 sec for header
  wait(8)
  // 11 * 2 = 22 sec for body
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.write('Hello World!');
      return wait(2);
    })
    .then(() => {
      res.end('Bye World!');
    });
});

app.listen(3001, () => {
  console.log('listening on port 3001!');
});