X-Powered-By: Este

gistfile1.js

// middleware.js
import onHeaders from 'on-headers';

export default function setupMiddleware(headers = [], options = {}) {
  
  return function onRequest(req, res, next) {
    
    // Do the action when headers are emitted for given res
    onHeaders(res, _ => {
      
      headers.forEach((value, header) => {
        
        // If header does not exist or keepExisting is true
        if (!res.getHeader(header) || !options.keepExisting) {
          res.setHeader(header, value);  
        }
        
      });
      
    });
    
    next();
  }
  
}

// main.js
import express from 'express';
import setupHeaders from './middleware';
 
const app = express();

app.use(setupHeaders({
  'X-Powered-By': 'Este.js',
  'X-Este-Version': '0.0.1'
}));

app.get('*', (req, res) => {
  res.end('Hello!');
});