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:

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

or with your preferred package manager:

`sh
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):

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

To ignore it
`ts
// biome-ignore lint/plugin/no-type-assertion:
const a = 5 as number;
`

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

$3

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

as syntax: expression as Type
`ts
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:
`typescript
// These are "uncommon" type assertions that the plugin at the moment cannot catch
const b = 5;
console.log((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:

$3

1. Clone the repository:
`sh
git clone https://github.com/albertodeago/biome-plugin-no-type-assertion.git
cd biome-plugin-no-type-assertion
`

2. Install dependencies:
`sh
npm install
`

$3

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

#### Testing

Run the test suite:

`sh
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:
`sh
npm publish
``
3. Tag the release in GitHub creating a Release with the changes.

License

MIT © Alberto De Agostini