Transparent MessagePack middleware for Express

Usage no npm install needed!

<script type="module">
  import expressMsgpack from '';


Express Msgpack

License Build Status Test Coverage Maintainability NPM Version

Express and MessagePack, together at last. Uses @msgpack/msgpack by default.


Provides transparent middleware that can be used to support clients requesting Accept: application/msgpack from endpoints using res.json or sending Content-Type: application/msgpack to any endpoint. You can continue to use req.body and res.json and expressMsgpack will handle the conversion in the background using @msgpack/msgpack (or any compatible library of your choice).


$ npm install --save express-msgpack
// or
$ yarn add express-msgpack

If you intend to use an alternative to @msgpack/msgpack (see Configuration) you can add the --no-optional flag; it's an optional dependency.


const { default: msgpack } = require("express-msgpack");

// ...


To configure, pass options when you configure the middleware. Currently supported options are:

Parameter Description Default
decoder a function converting from MessagePack to JavaScript @msgpack/msgpack#decode
encoder a function converting from JavaScript to MessagePack @msgpack/msgpack#encode (with a wrapper to convert the result to a Buffer)
mimeType the MIME type to detect and set for MessagePack payloads "application/msgpack"

For example, to switch to the node-gyp C++ based msgpack library:

const { pack, unpack } = require("msgpack");
const { default: msgpack } = require("express-msgpack");

// ...

app.use(msgpack({ decoder: unpack, encoder: pack }));


The project has code linting and testing, using the following commands:

  • npm run e2e: run the smoke/E2E tests
  • npm run lint: run the ESLint checks
  • npm run ship: lint and run unit, integration and E2E tests
  • npm test: run the Jest unit and integration tests
  • npm test:watch: run the tests in watch mode

The tests are in the __tests__/ directory and are run using Jest. They're split into two files:

  • unit.test.ts - mockist unit tests, to check specific internal details
  • integration.test.ts - integration tests using SuperTest with a simple Express app using the middleware

There is also a smoke/ directory containing E2E/smoke tests for a deployed version of the package, used by bin/ If the --local argument is supplied to the script the local version is packaged and tested , otherwise the specified $TAG version is installed from the registry and tested.