Changes of this fork

Support intercepting binary response. If the response is not in text, ( indicated by Content-Type ), The body passed by intercept( body ) will be a Buffer instance.
Support promise on isInterceptable
Use es6-promise to support Node v0.10

Follows

Done - ~~fix the failed tests~~

Updated usage

express().use(require('express-interceptor')(function(req, res){
  return {
    isInterceptable: function(){
      if (/^text\/html/.test(req.originalUrl)) {

        // Can return promise
        return new Promise(function(resolve){
          setTimeout(function(){
            resolve(true);
          }, 1000);
        });
      } else {

        // Or return boolean directly
        return false;
      }
    },
    intercept: function(body, send) {

      // Call .toString explicitly
      console.log(body.toString('utf-8'));

      send(body);
    }
  };
})

express-interceptor

A tiny Express response interceptor

Express-interceptor allows you to define a previous step before sending a response. This allows you to do anything you want with the response, such as processing, transforming, replacing, or logging it. Express-interceptor allows you to avoid calling next() over and over. Further more, you can avoid managing nested scopes. Using a declarative API, it’s simple to use and maintain.

Some use cases include:

Transpile custom elements into standard HTML elements.
Transform JSX or compile less files on the fly.
Store statistics about responses.
Set response headers based on tags in the response body.
Dynamically inject live-reload scripts to HTML.

Usage

Install the package

npm i --save express-interceptor

Define your interceptor

var express     = require('express');
var cheerio     = require('cheerio');
var interceptor = require('express-interceptor');

var app = express();

var finalParagraphInterceptor = interceptor(function(req, res){
  return {
    // Only HTML responses will be intercepted
    isInterceptable: function(){
      return /text\/html/.test(res.get('Content-Type'));
    },
    // Appends a paragraph at the end of the response body
    intercept: function(body, send) {
      var $document = cheerio.load(body);
      $document('body').append('<p>From interceptor!</p>');

      send($document.html());
    }
  };
})

// Add the interceptor middleware
app.use(finalParagraphInterceptor);

app.use(express.static(__dirname + '/public/'));

app.listen(3000);

You're defining an interceptor that will be executed whenever the response contains html as Content-Type. If this is true, it will append a child at the end of the body. In other words, it will transform the response:

<html>
<head></head>
<body>
  <p>"Hello world"</p>
</body>
</html>

Into:

<html>
<head></head>
<body>
  <p>"Hello world"</p>
  <p>From interceptor!</p>
</body>
</html>

See more examples. Also, you can debug express-interceptor actions using debug env variable DEBUG=express-interceptor.

API

isInterceptable() (required): is a predicate function where you define a condition whether or not to intercept a response. Returning true buffers the request, and proceeds calling intercept() as well as afterSend(). Typically, you want to check for this condition in the res object in the definition of the middleware.
intercept(body, send) (required): Parse the body as an encoded string. After processing the body, call send(newBody) with the content to be sent back to the client.
afterSend(oldBody, newBody): This method will be called after sending the response to the client – after the done() callback in the send() method is executed. This method would typically be used to cache something, log stats, fire a job, among other things.

Who is using it?

Similar to

express-hijackresponse (deprecated) Different API, using callbacks with no top down structure, more obtrusive to HTTP internals.
hijackresponse Attempting to solve same problems than express-hijackresponse. Different API using pipes, recommended if you have problems with streaming or if you need more control over responses.
tamper Similar functionality but different internals and API.

Words of advice

If your intercept method make calls to a database, or needs to make expensive transformations to the original response, you should take in account the time that will add to the response. You should define your isInterceptable method carefully, checking the type of the response, or even the route that was fired by the request, also you may consider implementing a cache strategy to get faster responses.

If you face any issue, don't hesitate to submit it here.

Contributing

Please check CONTRIBUTING.md.

This project adheres to the Open Code of Conduct. By participating, you are expected to honor this code.

fuminchao/express-interceptor