on-headers

What is on-headers?

The on-headers npm package is used to execute custom logic when headers are about to be written to the response object in a Node.js HTTP server. This can be useful for logging, setting default headers, or performing other actions based on the headers before they are sent to the client.

What are on-headers's main functionalities?

Execute custom logic before headers are sent

This code sample demonstrates how to use the on-headers package to execute a function right before the headers are sent in an HTTP response. The function `onHeadersListener` is called just before the headers are written to the response.

const onHeaders = require('on-headers');

function onHeadersListener(res) {
  console.log('Headers will be sent soon.');
  // Perform any custom logic here
}

http.createServer(function (req, res) {
  onHeaders(res, onHeadersListener);
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World!');
});

Other packages similar to on-headers

on-finished

The on-finished package is similar to on-headers in that it allows you to execute code based on HTTP request/response lifecycle events. However, on-finished is used to detect when the HTTP response has finished or when the request has been closed, rather than when headers are about to be sent.

prepend-http

While not directly similar to on-headers, prepend-http is another package that deals with HTTP headers. It is used to prepend `http://` to a URL if it's missing. This package does not provide hooks into the response object's lifecycle.

helmet

Helmet is a package that provides a set of middleware functions to set HTTP response headers for security purposes. It differs from on-headers as it focuses on security headers and provides a higher-level API for configuring them, rather than providing a hook for any kind of custom logic before headers are sent.

README

on-headers

Execute a listener when a response is about to write headers.

Installation

This is a Node.js module available through the npm registry. Installation is done using the npm install command:

$ npm install on-headers

API

var onHeaders = require('on-headers')

onHeaders(res, listener)

This will add the listener listener to fire when headers are emitted for res. The listener is passed the response object as it's context (this). Headers are considered to be emitted only once, right before they are sent to the client.

When this is called multiple times on the same res, the listeners are fired in the reverse order they were added.

Examples

var http = require('http')
var onHeaders = require('on-headers')

http
  .createServer(onRequest)
  .listen(3000)

function addPoweredBy () {
  // set if not set by end of request
  if (!this.getHeader('X-Powered-By')) {
    this.setHeader('X-Powered-By', 'Node.js')
  }
}

function onRequest (req, res) {
  onHeaders(res, addPoweredBy)

  res.setHeader('Content-Type', 'text/plain')
  res.end('hello!')
}

Testing

$ npm test

License

MIT