An ESLint plugin to support type-only files in TypeScript. Files with a matching pattern can only use types, interfaces and enums.

• Files that should only describe types - e.g. api.types.ts
• Safely excluding type-only files from test coverage
• In a monorepo, safely excluding changes to type-only modules from test / deploy invalidation

This plugin depends on @typescript-eslint/parser

yarn add --dev @typescript-eslint/parser eslint-plugin-type-only-files

Ensure the parser is set in your ESLint config (e.g. .eslintrc), then add the plugin:

{
  "parser": "@typescript-eslint/parser",
  "plugins": [
    "type-only-files"
  ],
  "rules": {
    "type-only-files/only-types": ["error"]
  }
}

The rule will default to banning non-types and non-type import/exports in files named *.types.ts(x) or types.ts(x).

Modify your configuration as desired (defaults are shown):

{
  "rules": {
    "type-only-files/only-types": [
      "error", {
        "banEnums": true,
        "filePattern": /[.\\/]types\.tsx?$/.source
      }
    ]
  }
}

Prohibit enums in type-only files.

Note that this can't detect when an enum is imported via import type { MyEnum }.

Specify the pattern for "type only files" in your project. By default, this is any file ending .types.ts[x]

Type-only files should only use type imports (e.g. "import type { }").

Rationale:

• Prevents a type-file performing side effects via the import statement

Banned:

import 'sideEffect'
import * as A from 'file'
import { valueA, valueB } from 'file'

Allowed:

import type * as T from 'file'
import type { TypeA, TypeB } from 'file'
import { type TypeA, type TypeB } from 'file'

Type-only files should only export types, interfaces, or enums.

Rationale:

• A type-file should only contribute types to a project
• A type-file should only re-export types, without side effects

Banned:

export const variable = 'a'
export default someExpression()

export { valueA, valueB }
export { type TypeA, valueB }

Allowed:

export type TypeA = { }

export type { TypeA, TypeB }
export { type TypeA, type TypeB }

export enum MyEnum { }

Limitation: this plugin cannot detect the case export { MyEnum } as a type-only export. Work around this with export enum MyEnum { ... }.

Type-only files should only declare types, interfaces, or enums. Found a VariableDeclaration.

Rationale:

• A type-file should only declare, export or import pure types
• If you need to do a typeof, export a type from a non-type file

This software is MIT licensed.

Please feel free to contribute changes and issues to the GitHub project. Pull requests are welcome.

Copyright Jimmy Breck-McKye 2023