Express Response Middleware

[!NOTE]
Come from express-mung? Checkout the Migration Guide

Middleware for express responses. Fork of express-mung

This package allows synchronous and asynchronous transformation of an express response. This is a similar concept to the express middleware for a request but for a response. Note that the middleware is executed in LIFO order. It is implemented by monkey patching (hooking) the res.end, res.json, or res.write methods.

Installation | Quick Start | API | Contributing

📦 Installation

NPM

npm i express-response-middleware

Yarn

yarn add express-response-middleware

PNPM

pnpm i express-response-middleware

🚀 Quick Start

Sample middleware (redact.js) to remove classified information.

import { jsonMiddleware } from 'express-response-middleware'

/* Remove any classified information from the response. */
export const hideSecretMiddleware = jsonMiddleware((body, req, res) => {
    if (body.secret) body.secret = '****'
    // ...
    return body
})

then add to your app.js file (before the route handling middleware)

app.use(hideSecretMiddleware)

💻 API

• Json Middleware
• Jsonp Middleware
• Send Middleware
• End Middleware
• Write Middleware
• Legacy docs
  • V1

jsonMiddleware

Intercept res.json, allow transform the JSON body of the response.

import { jsonMiddleware } from 'express-response-middleware'

const myMiddleware = jsonMiddleware((json, req, res) => {
    // your code here
    return json
})

jsonpMiddleware

Intercept res.jsonp, allow transform the JSON body of the response.

import { jsonpMiddleware } from 'express-response-middleware'

const myMiddleware = jsonpMiddleware((json, req, res) => {
    // your code here
    return json
})

sendMiddleware

Intercept res.send, allow transform the body of the response.

import { sendMiddleware } from 'express-response-middleware'

const myMiddleware = sendMiddleware((payload, req, res) => {
    // your code here
    return payload
})

endMiddleware

Intercept end.json, allow transform the HTTP headers of the response.

import { endMiddleware } from 'express-response-middleware'

const myMiddleware = endMiddleware((req, res) => {
    // your code here
})

[!CAUTION] Sending a response while in endMiddleware* is undefined behaviour and will most likely result in an error

writeMiddleware

Intercept end.write, allow transform buffer.

import { writeMiddleware } from 'express-response-middleware'

const myMiddleware = writeMiddleware((chunk, encoding, req, res) => {
    // your code here
    return chunk
})

[!CAUTION] Promise callback support is limited, it doesn't resolve multiple write call yet Code

• When writeMiddleware detects that a response has completed (i.e. if res.end has been called), it will abort.
• Calling res.json or res.send from writeMiddleware can lead to unexpected behavior since they end the response internally.
• The returned value of res.write will be inaccurate when using writeMiddleware, beware if you rely on it
• res.end after res.write would not trigger endMiddleware, as header already sent

Exception handling

responseMiddleware catches any exception (synchronous, asynchronous or Promise reject) and sends an HTTP 500 response with the exception message. You should handle your own error if you want different behavior

🤝 Contributing

TODO

Current state project is up to modern standard and support all of the express-mung use case, here is the list that I think can improve on.

• [ ] Support multiple write call with writeMiddleware + async handler

Development

Local Development

pnpm i
pnpm test

Build

pnpm build

Release

This repo uses Release Please to release.

To release a new version

1. Merge your changes into the main branch.
2. An automated GitHub Action will run, triggering the creation of a Release PR.
3. Merge the release PR.
4. Wait for the second GitHub Action to run automatically.
5. Congratulations, you're all set!