ts-inference-check

Make sure TypeScript is making the expected type inference from your types.

npm i ts-inference-check --save-dev

Usage

👉 Check if inferred type of some obj is SomeType (and is not SomeOtherType):

import { type } from 'ts-inference-check'


type(obj).is<SomeType>(true)
type(obj).is<SomeOtherType>(false)

☝️ type(...).is<T>(true) will result in a compiler error if inferred type is not T. Similarly, type(...).is<T>(false) will result in a compiler error if inferred type is T.

Example usage in Jest (you can use with any testing library):

import { type } from 'ts-inference-check'

test('something', () => {
  // ...

  expect(type(obj).is<SomeType>(true)).toBe(true)
  expect(type(obj).is<SomeOtherType>(false)).toBe(false)

  // ...
})

💡 type(...).is<T>() only accepts true if inferred type is T, and only accepts false if inferred type is not T. It will return the given value untouched. All other methods of type(...) or type<T>() behave in the same manner.

👉 Check if inferred type is subtype of SomeType (and is not subtype of SomeOtherType):

type(obj).extends<SomeType>(true)  
type(obj).extends<SomeOtherType>(false)

👉 Check if inferred type is super type of SomeType (and is not supertype of SomeOtherType):

type(obj).isExtendedBy<SomeType>(true)
type(obj).isExtendedBy<SomeOtherType>(false)

👉 Pass type arguments instead of values:

type<A>().is<B>(true)
type<A>().extends<B>(true)
type<A>().isExtendedBy<B>(true)

Strict Checks

👉 any passes most of the checks described above. Use type(...).strictly to avoid this:

type(obj).strictly.is<SomeType>(true)
type(obj as any).is<SomeType>(true)
type(obj as any).strictly.is<SomeType>(false)

type(obj as any).extends<SomeType>(true)
type(obj as any).strictly.extends<SomeType>(false)
type(obj as any).isExtendedBy<SomeType>(true)
type(obj as any).strictly.isExtendedBy<SomeType>(false)

👉 Use .isAny() to check if inferred type is any:

type(obj as any).isAny(true)
type(obj).isAny(false)

Contribution

Simply be nice and respectful to other people. Here are some useful commands for contributing:

git clone git@github.com:loreanvictor/ts-inference-check.git  # 👉 clone the code (generally fork it before cloning though)

npm i            # 👉 install all dependencies

npm test         # 👉 run the tests

npm run coverage # 👉 check code coverage

npm run lint     # 👉 check code style

npm run lint:fix # 👉 fix trivial stylistic issues

When making a PR, make sure all checks pass. Specifically, run tests, check coverage and check the linter to ensure they all pass.