How to Create a Loader

In this document, you’ll learn how to create a loader in Medusa. A loader can be created in the Medusa backend codebase, in a plugin, or in a module.

Step 1: Create Loader File

Create a TypeScript or JavaScript file in the src/loaders directory that will hold your custom script. There are no restrictions on the name of the file.

For example, create the file src/loaders/my-loader.ts that will hold the loader.

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Step 2: Define the Loader

The loader file must export a function.

Parameters of Loaders in Medusa Backend and Plugins

When the loader is defined in the Medusa backend or a plugin, the function receives the following parameters:

1. container: the first parameter, which is a MedusaContainer object. You can use the container to register custom resources into the dependency container or resolve resources from the dependency container.
2. config: the second parameter, which is an object that holds the loader’s plugin options. If the loader is not created inside a plugin, the config object will be empty.

Parameters of Loaders in Modules

When the loader is defined in a module, it receives the following parameters:

1. container: the first parameter, which is a MedusaContainer object. You can use the container to register custom resources into the dependency container or resolve resources from the dependency container.
2. logger: the second parameter, which is a Logger object. The logger can be used to log messages in the console.
3. config: the third parameter, which is an object that holds the loader’s module options.

Example Implementation

For example, this loader function resolves the ProductService and logs in the console the count of products in the Medusa backend:

src/loaders/my-loader.ts

import { 
  ProductService, 
  ConfigModule,
  MedusaContainer,
} from "@medusajs/medusa"

export default async (
  container: MedusaContainer,
  config: ConfigModule
): Promise<void> => {
  console.info("Starting loader...")
  const productService = container.resolve<ProductService>(
    "productService"
  )
  console.info(`Products count: ${
    await productService.count()
  }`)
  console.info("Ending loader...")
}

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Step 3: Run Build Command

In the directory of your project, run the following command to transpile the files from the src to the dist directory:

• npm
• Yarn
• pnpm

npm run build

yarn build

pnpm run build

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Test it Out

Note

This section explains how to test out the loader if it’s created in the Medusa backend codebase. If you’re creating your loader in a plugin, you can learn how to test it in the plugins documentation. Alternatively, if you’re creating your loader in a module, you can learn how to test it in the modules documentation.

Run the following command to start the Medusa backend:

• npm
• Yarn
• pnpm

npx medusa develop

npx medusa develop

npx medusa develop

Your loader script should run on the Medusa backend startup. If you logged a message in the console, similar to the example above, you should see it in the console.