Module Boundaries

There are two types of modules: Barrel Modules, which include an index.ts file in their root folder, and Barrel-less Modules. Barrel-less modules require a configuration file, while barrel modules do not.

The recommendation is for barrel-less modules because they optimize tree-shaking.

A project allows both module types. However, if a module, configured as barrel-less, contains a barrel file, it becomes a barrel module.

Barrel-less Modules

Barrel-less modules have a subdirectory internal. All files in that subdirectory internal are encapsulated, i.e. other modules cannot access them.

The configuration file sheriff.config.ts defines these modules. The CLI command npx sheriff init generates the configuration automatically with the following content:

export const config: SheriffConfig = {
  modules: {},
  depRules: {
    root: 'noTag',
    noTag: 'noTag',
  },
};

The depRules can stay as they are. They allow all modules to access each other.

The modules object defines the modules. The key is a directory path relative to the project root. The value is a string or an array of strings, defining the tags of the module.

For example, the current project has the directories db and web. modules defines them as follows:

export const config: SheriffConfig = {
  modules: {
    db: 'noTag',
    web: 'noTag'
  },
  enableBarrelLess: true, // <-- this is important
  depRules: {
    root: "noTag",
    noTag: "noTag"
  }
};

Again, the value noTag means that there is no restriction on which modules can access each other.

The web module has a dependency on db. The file fetcher.ts in web imports db.ts from db. This is a valid import because db.ts is not located in the internal directory of db.

Therefore, ESLint does not show any errors.

However, if fetcher.ts accesses credentials.ts from db, ESLint will show an error. This results in an encapsulation violation because credentials.ts is located in the internal directory of db.

Barrel Modules

Barrel modules have an index.ts in their root folder. Sheriff detects them automatically, even if modules in sheriff.config.ts doesn't define them.

The index.ts file exports the files that other modules can access. The files that are not exported are encapsulated.

Since Sheriff detects them automatically, no configuration file is necessary. However, if a sheriff.config.ts exists, the initial content from the CLI is enough.

export const config: SheriffConfig = {
  depRules: {
    root: 'noTag',
    noTag: 'noTag',
  },
};

The screenshot below shows the same example with db and web as barrel modules. db has an index.ts that exports db.ts. credential.ts is not in an internal folder but is still encapsulated because it is not exported.

fetcher.ts accessing db.ts causes therefore no error.

An access to the non-exported credential.ts causes an error.

It is also possible to disable the automatic module detection. For more information, see Dependency Rules.

Barrel-less Modules​

Barrel Modules​

Barrel-less Modules

Barrel Modules