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How to Create a Plugin

In this document, you’ll learn how to create a plugin and some tips for develoment. If you’re interested to learn more about what plugins are and where to find available official and community plugins, check out the overview document.

Prerequisites​

This guide uses the Medusa CLI throughout different steps. If you don’t have the Medusa CLI installed you can install it with the following command:

npm install @medusajs/medusa-cli -g

If you run into any errors while installing the CLI tool, check out the troubleshooting guide.

Initialize Project​

The recommended way to create a plugin is using the Medusa CLI. Run the following command to create a new Medusa project:

medusa new medusa-plugin-custom

Where medusa-plugin-custom is the name of the plugin you’re creating. In Medusa, plugins are named based on their functionalities.

By convention, all plugin names start with medusa followed by a descriptive name of what the plugin does. For example, the Stripe plugin is named medusa-payment-stripe.

Changes to package.json​

Change Dependencies​

A basic Medusa backend installed with the medusa new command has dependencies similar to this:

package.json
"dependencies": {
    "@babel/preset-typescript": "^7.21.4",
    "@medusajs/cache-inmemory": "^1.8.0",
    "@medusajs/cache-redis": "^1.8.0",
    "@medusajs/event-bus-local": "^1.8.0",
    "@medusajs/event-bus-redis": "^1.8.0",
    "@medusajs/medusa": "^1.8.0",
    "@medusajs/medusa-cli": "^1.3.9",
    "babel-preset-medusa-package": "^1.1.13",
    "body-parser": "^1.19.0",
    "cors": "^2.8.5",
    "express": "^4.17.2",
    "medusa-fulfillment-manual": "^1.1.37",
    "medusa-interfaces": "^1.3.7",
    "medusa-payment-manual": "^1.0.23",
    "medusa-payment-stripe": "^2.0.0",
    "typeorm": "^0.3.11"
  },
  "devDependencies": {
    "@babel/cli": "^7.14.3",
    "@babel/core": "^7.14.3",
    "@types/express": "^4.17.13",
    "@types/jest": "^27.4.0",
    "@types/node": "^17.0.8",
    "babel-preset-medusa-package": "^1.1.13",
    "cross-env": "^5.2.1",
    "eslint": "^6.8.0",
    "jest": "^27.3.1",
    "mongoose": "^5.13.14",
    "rimraf": "^3.0.2",
    "ts-jest": "^27.0.7",
    "ts-loader": "^9.2.6",
    "typescript": "^4.5.2"
  },

For a plugin, some dependencies are not necessary. For example, can remove the packages medusa-fulfillment-manual, medusa-payment-manual, and medusa-payment-stripe as they are fulfillment and payment plugins necessary for a Medusa backend, but not for a plugin. The same goes for modules like @medusajs/cache-inmemory.

Additionally, you can remove @medusajs/medusa-cli as you don’t need to use the Medusa CLI while developing a plugin.

Once you’re done making these changes, re-run the install command to update your node_modules directory:

npm install

It's recommended to remove the seed and start scripts from your package.json as they aren't necessary for plugin development.

Furthermore, if you don't have a watch command in your package.json it's recommended to add it:

package.json
"scripts": {
  // other scripts...
  "watch": "tsc --watch"
}

The watch command makes the testing of the plugin easier.

The watch command outputs the files in the destination specified in the value of outDir in tsconfig.json, and the same goes for the build command. If you made changes to tsconfig.json, make sure the destination is either the dist directory or the root of the plugin. You can learn more in the plugin structure section.

Develop your Plugin​

Now, You can start developing your plugin. This can include adding services, endpoints, entities, or anything that's relevant to your plugin.

Plugin Structure​

While developing your plugin, you can create your TypeScript or JavaScript files under the src directory. This includes creating services, endpoints, migrations, and other resources.

However, before you test the changes on a Medusa backend or publish your plugin, you must transpile your files and move them either to a dist directory or to the root of the plugin's directory.

For example, if you have an endpoint in src/api/index.js, after running the build or watch commands as defined earlier, the file should be transpiled into dist/api/index.js in your plugin's root. You can alternative transpile them into the api/index.js in your plugin's root.

It was previously required to output your files into the root of the plugin's directory (for example, api/index.js instead of dist/api/index.js). As of v1.8, you can either have your files in the root of the directory or under the dist directory.

Development Resources​

This guide doesn't cover how to create different files and components. If you’re interested in learning how to do that, you can check out these guides:

Add Plugin Configuration​

Plugins often allow developers that will later use them to enter their own configuration. For example, you can allow developers to specify the API key of a service you’re integrating.

To pass a plugin its configurations on a Medusa backend, you have to add it to the plugins array in medusa-config.js:

medusa-config.js
const plugins = [
  // ...
  {
    resolve: `medusa-plugin-custom`,
    options: {
      name: "My Store",
    },
  },
]

Then, you can have access to your plugin configuration in the constructor of services in your plugin:

src/service/test.ts
  // In a service in your plugin
class MyService extends TransactionBaseService {
  constructor(container, options) {
    super(container)
    // options contains plugin configurations
    this.name = options.name
  }
  // ...
}

You can also have access to the configurations in endpoints in your plugin:

src/api/index.ts
// in an endpoint in your plugin
export default (rootDirectory, options) => {
  // options contain the plugin configurations
  const router = Router()

  router.get("/hello-world", (req, res) => {
    res.json({
      message: 
      `Welcome to ${options.name ? options.name : "Medusa"}!`,
    })
  })

  return router
}

Make sure to include in the README of your plugin the configurations that can be passed to a plugin.

Test Your Plugin​

While you develop your plugin, you’ll need to test it on an actual Medusa backend. This can be done by using the npm link command.

In the root of your plugin directory, run the following command:

npm link

Then, change to the directory of the Medusa backend you want to test the plugin on and run the following command:

npm link medusa-plugin-custom

Where medusa-plugin-custom is the package name of your plugin.

After linking to your plugin in a local Medusa backend, either run the build or watch commands in your plugin directory:

# in the directory of the plugin
npm run watch

If you’re running the watch command, you don’t need to run the build command every time you make a change to your plugin.

Then, add your plugin into the array of plugins in medusa-config.js:

medusa-config.js
const plugins = [
  // ...
  {
    resolve: `medusa-plugin-custom`,
    // if your plugin has configurations
    options: {
      name: "My Store",
    },
  },
]

If your plugin has migrations, you must run them before you start the backend. Check out the Migrations guide for more details.

Finally, start your backend and test your plugin’s functionalities:

npm run start

Troubleshoot Errors​

Error: The class must be a valid service implementation​

Please make sure that your plugin is following the correct structure. If the error persists then please try the following fix:

cd <BACKEND_PATH>/node_modules/medusa-interfaces
npm link
cd <BACKEND_PATH>/node_modules/@medusajs/medusa
npm link
cd <PLUGIN_PATH>
rm -rf node_modules/medusa-interfaces
rm -rf node_modules/@medusajs/medusa
npm link medusa-interfaces
npm link @medusajs/medusa
npm link
cd <BACKEND_PATH>
npm link your-plugin

Where <BACKEND_PATH> is the path to your Medusa backend and <PLUGIN_PATH> is the path to your plugin.

This links the medusa-interfaces and @medusajs/medusa packages from your medusa-backend to your plugin directory and then links your plugin to your medusa-backend.

APIs not loading​

If the APIs you added to your Medussa backend are not loading then please try the following steps:

cd <PLUGIN_PATH>
rm -rf node_modules
cd <BACKEND_PATH>/node_modules/<PLUGIN_NAME>
npm install
cd <PLUGIN_PATH>
npm run build
cd <BACKEND_PATH>
npm run start

Where <BACKEND_PATH> is the path to your Medusa backend, <PLUGIN_PATH> is the path to your plugin and <PLUGIN_NAME> is the name of your plugin as it is in your plugin package.json file.

It is safe to ignore any cross-env: command not found error you may receive.

Publish Plugin​

Once you're done with the development of the plugin, you can publish it to NPM so that other Medusa developers and users can use it.

Please refer to this guide on required steps to publish a plugin.