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jest-runner-tsd

Run your TypeScript type tests using Jest.

version license

Note: the jest-runner-tsd is using tsd-lite instead of tsd. Both of them have the same type testing logic, but tsd-light makes it easier to test projects written in TypeScript (or types generated by your library).

Most important differences (for the full list see tsd-lite repo):

  • tsd-lite has no additional rules or checks;
  • jest.config is used to discover test files;
  • and tsconfig.json provides configuration for TS compiler. For details see Configuration section.

Install

yarn add -D jest-runner-tsd @tsd/typescript

Remember to install @tsd/typescript package. It is a required peer dependency.

Configuration

Jest

First of all, you should configure Jest to discover your test files. For example, if the files are suffix with .test.ts and live inside __typetests__ directory, set up jest.config.tsd.js like this:

module.exports = {
  displayName: {
    color: 'blue',
    name: 'types',
  },
  runner: 'jest-runner-tsd',
  testMatch: ['**/__typetests__/*.test.ts'],
};

TS Compiler

Your test files will be compiled using the TypeScript compiler (similarly to tsc), but, instead of emitting JS code, tsd-lite will analyze types and diagnostics returned by the compiler.

To compile each test file, tsd-lite will read the nearest tsconfig.json and will pass the configuration to the compiler. Hence, you may have a configuration for the whole project, or a group of test files, or just a particular test file.

For example, if your project already includes a tsconfig.json in the root directory, but you prefer to have different configuration for testing, simply add another tsconfig.json to a directory with the test files. It may override or extend your root configuration.

Hint: run yarn tsc -p path/to/__typetests__ --showConfig to print the configuration which applies to the test files.

Note: if tsconfig.json is not found, the compiler will fall back to the default configuration.

Optionally Strict

Just like TypeScript, tsd-lite is optionally strict. In contrary, the vanilla tsd is strict by default. If you are migrating your test suite, remember to set "strict": true in tsconfig.json.

import { expectType } from 'tsd-lite';

declare const loggedInUsername: string;

const users = [
  { name: 'Oby', age: 12 },
  { name: 'Heera', age: 32 },
];

const loggedInUser = users.find(u => u.name === loggedInUsername);

expectType<number>(loggedInUser.age);

The assertion in this example fails with "strict": true, but passes with "strict": false.

Running Tests

If all is set, simply run yarn jest -c jest.config.tsd.js command. Or better include a script in package.json:

"scripts": {
  "test:types": "jest -c jest.config.tsd.js"
}

Learn More

To learn more about tsd and its assertions see the documentation.

License

MIT © Jest Community