v4 code migration: Updating services

This guide is part of the v4 code migration guide designed to help you migrate the code of a Strapi application from v3.6.x to v4.0.x

🤓 v3/v4 comparison

In both Strapi v3 and v4, creating content-types automatically generates core API services. Services are a set of reusable functions.

In Strapi v3, services export an object containing actions that are merged with the existing actions of core API services, allowing customization.

In Strapi v4, services export the result of a call to the createCoreService factory function, with or without further customization.

Migrating a service to Strapi v4 consists in making sure it uses the createCoreService factory function introduced in v4.

Due to the differences between services implementation in Strapi v3 and v4, it's recommended to create a new service file, then optionally bring existing v3 customizations into the new file and adapt them when necessary.

✏️ Note

The service file can be created automatically with the interactive CLI command strapi generate.

To create a Strapi v4 service:

Create a api/<api-name>/services/<service-name>.js file inside the ./src folder (see project structure).
Copy and paste the following code at the top of the ./src/api/<api-name>/services/<service-name>.js file. The code imports the createCoreService factory function from the factories included with the core of Strapi:
```
const { createCoreService } = require('@strapi/strapi').factories;
```
Copy and paste the following code, replacing api-name and content-type-name with appropriate names. The code exports the result of a call to the createCoreService factory function, passing the unique identifier of the content-type (e.g. api::api-name.content-type-name) as an argument:
```
module.exports = createCoreService('api::api-name.content-type-name')
```
(optional) To customize the service, pass a second argument to the createCoreService factory function. This argument can be either an object or a function returning an object. The object contains methods, which can either be entirely new actions or replace or extend existing actions of core API controllers (see services implementation documentation).

Example of a v4 service without customization:

./src/api/<content-type-name>/services/<service-name>.js

const { createCoreService } = require('@strapi/strapi').factories;

module.exports = createCoreService('api::api-name.content-type-name');

Example of a v4 service with customization:

./src/api/<content-type-name>/services/<service-name>.js

const { createCoreService } = require('@strapi/strapi').factories;

module.exports = createCoreService('api::api-name.content-type-name', ({ strapi }) => ({
  async find(...args) {
    const { results, pagination } = await super.find(...args);

    results.forEach(result => {
      result.counter = 1;
    });

    return { results, pagination };
  },
}));

💡 Customization tips

The original service’s CRUD methods can be called using super (e.g. super.find()).
The this.getFetchParams() utility function can be used to wrap parameters with some default parameters (e.g. a default value for the publicationState). this.getFetchParams() has the following signature: (params: Object) => Object.

More examples can be found in the services implementation documentation.

🤓 Next steps

Migrating the backend code of Strapi to v4 also requires to at least migrate the core features of the Strapi server, such as the configuration, dependencies, routes, controllers, and content-type schemas.