biome-plugin-no-type-assertion

A Biome linter plugin that disallows type assertions in TypeScript code, encouraging the use of type guards and proper typing instead.

Installation

Install the plugin as a dev dependency:

npm install --save-dev biome-plugin-no-type-assertion

or with your preferred package manager:

yarn add -D biome-plugin-no-type-assertion
pnpm add -D biome-plugin-no-type-assertion
bun add -D biome-plugin-no-type-assertion

Usage

Add the plugin to your Biome configuration file (biome.json or biome.jsonc):

{
  "$schema": "./node_modules/@biomejs/biome/configuration_schema.json",
  "plugins": ["node_modules/biome-plugin-no-type-assertion/no-type-assertion.grit"]
}

To ignore it

// biome-ignore lint/plugin/no-type-assertion: <reason>
const a = 5 as number;

The plugin will now check your TypeScript code and report errors when it detects type assertions.

What it catches

The plugin detects only one form of type assertions in TypeScript (the most used):

as syntax: expression as Type

const a = 5 as number; // ❌ Error
const b = ["nope"] as string[]; // ❌ Error

Unfortunately, due to limitations in the Grit pattern language, the plugin cannot currently detect type assertions using the angle-bracket syntax. Examples of unsupported cases include:

// These are "uncommon" type assertions that the plugin at the moment cannot catch
const b = <number>5;
console.log(<string>(<unknown>b));

Why avoid type assertions?

Type assertions bypass TypeScript's type checker and can hide potential bugs. Instead, consider:

• Type guards: Use runtime checks with type predicates
• Proper typing: Define correct types at the source
• Type narrowing: Let TypeScript infer types through control flow analysis

Contributing

Contributions are welcome! Here's how to get started:

Setup

1. Clone the repository:

   git clone https://github.com/albertodeago/biome-plugin-no-type-assertion.git
   cd biome-plugin-no-type-assertion

2. Install dependencies:

   npm install

Development

The plugin is defined in no-type-assertion.grit using the Grit pattern language.

Testing

Run the test suite:

npm test

Tests are located in the test directory and use Vitest. Test fixtures can be found in test/fixtures.

Publishing

To publish a new version of the plugin:

1. Update the version in package.json.
2. Publish to npm:

   npm publish

3. Tag the release in GitHub creating a Release with the changes.

License

MIT © Alberto De Agostini