hijackresponse

Module that allows you to rewrite HTTP responses from middleware further down the stack, such as static providers, HTTP proxies etc.

Requires node v8 or later.

This module is the spiritual successor to express-hijackresponse attempting to solve the same goals. The differences from the original module are primarily that the API is slightly different, there's no direct coupling to express and it supports streams2.

It's mostly useful for content filters. The original use case is injecting an inline JavaScript into all HTML responses in LiveStyle. It is also used in a series of transpiler and preprocessing middleware:

• express-compiless
• express-processimage
• express-extractheaders
• express-autoprefixer

Installation

$ npm install hijackresponse

Usage

var express = require("express");
var hijackResponse = require("hijackresponse");

var app = express();

app.use((req, res, next) => {
  hijackResponse(res, next).then(({ readable, writable }) => {
    // Don't hijack HTML responses:
    if (/^text\/html/.test(res.getHeader("Content-Type"))) {
      return readable.pipe(writable);
    }

    res.setHeader("X-Hijacked", "yes!");
    res.removeHeader("Content-Length");

    readable.pipe(transformStream).pipe(writable);
  });
});

API

Function hijackResponse()

hijackResponse(res[, cb]) => Promise<HijackedReponse>

The hijackResponse function takes one required argument - the response object which is the target of the hijacking. The second optional argument, is a callback to be called when the hijacking preparations are done; this will mostly be used when you are working with express. You can also decide to call the callback afterwards if you prefer. The following two examples are equivalent:

app.use((req, res, next) => {
  hijackResponse(res, next).then(() => { /* ... */});
});

app.use((req, res, next) => {
  hijackResponse(res).then(() => { /* ... */});
  next();
});

The first example is easier to work with when you are working with async/await:

// Using express-promise-router or equivalent

app.use(async (req, res, next) => {
  const hijackedResponse = await hijackResponse(res, next);

  // ... do something with the hijacked reponse.
})

Object hijackedResponse

{
  readable: NodeJS ReadableStream,
  writable: NodeJS Writable,
  destroyAndRestore: Function
}

The resolution value of the Promise returned from calling hijackResponse.

• readable is a readable stream containing the captured response body.
• writable is a writable stream which will be sent to the client.
• destroyAndRestore is a function that destroys the readable stream, and restores the original res.

Everything written to res in other handlers are captured, so if you want to delegate to the express errorhandler you need to call destroyAndRestore before doing so. Calling destroyAndRestore will undo the hijack, and destroy the readable stream, meaning that all data written to it so far is discarded.

app.use((req, res, next) => {
  hijackResponse(res, next).then((hijackedResponse) => {
    hijackedResponse.destroyAndRestore();
    return next(new Error('Something bad happened'));
  });
});

If you don't call destroyAndRestore before passing the error to next, the errorhandlers output will become available on the readable-stream instead of being sent to the client as intended.

License

This module is published under the ISC License. See the LICENCE file for additional details.