eslint-plugin-expect-type

ESLint plugin with $ExpectType and $ExpectError type assertions

Installation

Make sure you have TypeScript and @typescript-eslint/parser installed, then install the plugin:

npm i -D eslint-plugin-expect-type

Please also make sure the following packages are also installed:

npm i -D eslint @typescript-eslint/eslint-plugin @typescript-eslint/parser

Usage

Please add the following options to your .eslintrc

{
  "extends": ["plugin:eslint-plugin-expect-type/recommended"],
  "parser": "@typescript-eslint/parser",
  "parserOptions": {
    "project": "./tsconfig.json"
  },
  "plugins": ["eslint-plugin-expect-type"],
  "root": true
}

Rule severity could be configured as follows

{
  "rules": {
    "expect-type/expect": "error"
  }
}

To skip Snapshot update when eslint is run with --fix could be configured as follows:

{
  "rules": {
    "expect-type/expect": ["error", { "disableExpectTypeSnapshotFix": true }]
  }
}

Note: Make sure to use eslint --ext .js,.ts since by default eslint will only search for .js files.

Adding $ExpectType and $ExpectError type assertions

A test file should be a piece of sample code that tests using the library. Tests are type-checked, but not run. To assert that an expression is of a given type, use $ExpectType. To assert that an expression causes a compile error, use $ExpectError. (Assertions will be checked by the expect lint rule.)

(https://github.com/Microsoft/dtslint#write-tests)

import foo from 'lib-to-test'; // foo is (n: number) => void

// $ExpectType void
foo(1);

// Can also write the assertion on the same line.
foo(2); // $ExpectType void

// $ExpectError
foo('bar');

Adding $ExpectTypeSnapshot

Uses snapshot saved in file as expected type for expression.

Example:

foo.test.ts

// $ExpectTypeSnapshot MyFooSnapshot
const Foo = {
  a: 1,
  n: 17,
} as const;

By running eslint --fix the following file will be created in the folder of foo.test.ts:

__type-snapshots__/foo.test.ts.snap.json

By running eslint snapshot type will be matched with actual type and Error will be emitted in case types don't match.

To create/update snapshots

eslint --fix

References

1. https://github.com/gcanti/dtslint
2. https://github.com/Microsoft/dtslint
3. https://github.com/SamVerschueren/tsd

Appreciation

Many thanks to @ibezkrovnyi for creating the initial version and core infrastructure of this package! 💖