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A connect middleware to inject content into any response.
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A middleware to inject content into any HTTP response.

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Getting Started

Install the module with: npm install connect-injector


The basic API looks like this:

var injector = require('connect-injector');
var middleware = injector(function when(req, res) {
  // for this request and repsonse
  // return whether or not to enable injecting
}, function converter(content, req, res, callback) {
  content // the entire response buffer
  req // the HTTP request
  res // the HTTP response
  callback // (error, data) with the injected data

And can be used like any other Connect and Express middleware. You need to make sure to use the injector middleware before the actual content is being written.


JSONP support

A very useful example for connect-injector is to add JSONP support to any application/json repsonse:

var injector = require('connect-injector');
var inject = injector(function(req, res) {
  var isJSON = res.getHeader('content-type').indexOf('application/json') !== -1;
  return isJSON && req.query.callback;
}, function(data, req, res, callback) {
  callback(null, req.query.callback + '(' + data.toString() + ')');

// inject needs to be used before any middleware that writes to the response
connect().use(connect.query()).use(inject).use(/* your other middleware here */);

Now any application/json response will be wrapped into a callback if given the callback=xyz query parameter.

Code minification

Another use case would be to minify JavaScript files on the fly using UglifyJS:

var injector = require('connect-injector');
var connect = require('connect');
var UglifyJS = require('uglify-js');
// Cache for already uglified files
var cache = {};
// Function that uglifies JavaScript code
var uglify = function (code) {
  var toplevel = UglifyJS.parse(code);


  var compressor = UglifyJS.Compressor({
    warnings: false
  var compressed = toplevel.transform(compressor);


  return compressed.print_to_string();

var inject = injector(function(req, res) {
  return res.getHeader('content-type').indexOf('application/javascript') !== -1;
}, function(data, req, res, callback) {
  // Check the cache, uglify the code if not and add it
  if(!cache[req.url]) {
    cache[req.url] = uglify(data.toString());

  callback(null, cache[req.url]);

// inject needs to be used before any middleware that writes to the response
var app = connect().use(inject).use(connect.static(__dirname + '/../test'));


Rewriting proxied files

connect-injector is tested to work with http-proxy you can rewrite existing proxied content:

var httpProxy = require('http-proxy');
var connect = require('connect');
var injector = require('connect-injector');
var proxy = httpProxy.createProxyServer();
var inject = injector(function(req, res) {
  return res.getHeader('content-type').indexOf('text/html') === 0;
}, function(data, req, res, callback) {
  callback(null, data.toString().replace('</body>', '<p>Powered by connect-injector</p></body>'));
var proxyMiddleware = function(req, res) {
  // You need to rewrite your host in order to be able to hit virtual hosts = '';
  proxy.web(req, res, {
    target: ''

var proxyApp = connect().use(inject).use(proxyMiddleware);


After starting the server, check http://localhost:8080/connect-injector/dummycontent.html to see the injected content.


Copyright (c) 2016 David Luecke
Licensed under the MIT license.

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