TypeScript type assertion plugin for vitest.

📌 This plugin is in alpha version, and will probably stay that way for a long time, it lacks tests (a bit ironic) and real documentation! But I can't stop myself from publishing it. Moreover this plugin is a succession of tricks to integrate it to vitest (it would be incredible that vitest offer an official plugin system 💜). But it seems to work for my use cases and maybe for you too? I'm thinking of continuing to evolve it if I need it or if others start using it. Feel free to contribute or give feedback if you encounter any issues. Thank you.

Installation

Install the dependencies.

pnpm add -D vitest typescript vite-plugin-vitest-typescript-assert
# yarn and npm also works

Setup the plugin in vitest.config.ts file.

import { defineConfig } from 'vitest/config';
import { vitestTypescriptAssertPlugin } from 'vite-plugin-vitest-typescript-assert';

export default defineConfig({
  plugins: [vitestTypescriptAssertPlugin()],
});

Assertions APIs

This plugin was more than inspired by the tsd project, much of the assertion logic comes from their code.

That's why two APIs are available:

// Exactly the same behaviour as tsd (plus any bugs I might have introduced 🙈)
import * as tsd from 'vite-plugin-vitest-typescript-assert/tsd';

// A more descriptive/flexible API with some additions.
import * as tssert from 'vite-plugin-vitest-typescript-assert/tssert';

Named imports, alias imports and named exports are supported in both API.

import { expectType } from 'vite-plugin-vitest-typescript-assert/tsd';
import { expectType as assertType } from 'vite-plugin-vitest-typescript-assert/tsd';

export { expectType as assertType } from 'vite-plugin-vitest-typescript-assert/tsd';

tsd docs

expectType<ExpectedType>(value);
expectNotType<ExpectedType>(value);
expectAssignable<ExpectedType>(value);
expectNotAssignable<ExpectedType>(value);
expectError(value);
expectDeprecated(expression);
expectNotDeprecated(expression);
printType(expression);

tssert

tssert supports 4 signatures for each method. You can either specify the type in the generic or the value in the first argument, but never both at the same time. But don't worry, if this happens the plugin will send you an error.

expectType<string>().assignableTo<string>();
expectType<string>().assignableTo('nyan');
expectType('nyan').assignableTo<string>();
expectType('nyan').assignableTo('nyan');

expectType<ExpectedType>().assignableTo(value);
expectType<ExpectedType>().identicalTo(value);
expectType<ExpectedType>().subtypeOf(value);
expectType<ExpectedType>().equalTo(value);
expectType<ExpectedType>().toBeDeprecated();
expectType<ExpectedType>().toThrowError();
expectType<ExpectedType>().toThrowError(code);
expectType<ExpectedType>().toThrowError(message);

expectType<ExpectedType>().not.assignableTo(value);
expectType<ExpectedType>().not.identicalTo(value);
expectType<ExpectedType>().not.subtypeOf(value);
expectType<ExpectedType>().not.equalTo(value);
expectType<ExpectedType>().not.toBeDeprecated();

Options

vitestTypescriptAssertPlugin(options?: PluginOptions);

By default the plugin applies the following configuration.

const defaultOptions: PluginOptions = {
  report: ['type-error', 'type-assertion'],
  include: ['**/*.test.ts'],
  exclude: [],
  typescript: {
    configName: 'tsconfig.json',
    searchPath: process.cwd(),
    compilerOptions: {},
  },
};

• type-error: Report the errors raised by the TS compiler.
• type-assertion: Report the errors raised by the assertion library.

export interface PluginOptions {
  report?: 'type-error' | 'type-assertion';
  include?: string | string[];
  exclude?: string | string[];
  typescript?: TypeScriptConfigOptions;
}

interface TypeScriptConfigOptions {
  configName?: string;
  searchPath?: string;
  compilerOptions?: ts.CompilerOptions;
}

Contributing 💜

See CONTRIBUTING.md