expect.extend: not possible to correctly extend corresponding TypeScript types

[NaridaL]

🐛 Bug Report

When extending expect

import expect from "expect"
expect.extend({
  toWorkProperly(received: number, actual: number) {
    return {pass:true}
  }
})

it should be possible to extend the corresponding types:

declare module "expect" {
  interface Matchers<R> {
    toWorkProperly(actual: number): void
  }
}

instead you get

To Reproduce

see above

Expected behavior

There should be no error and proper typechecking.

The reason for the issue, is that expect doesn't actually export the Matchers interface which expect() returns, but rather a sub-interface. See https://github.com/facebook/jest/blob/master/packages/expect/src/index.ts#L429 .

Unfortunately, it isn't possible to do the right thing directly here, i.e., you can't re-export an interface in a namespace. The quick option would be to move the Matchers declaration directly into the namespace instead of importing it from './types'. The proper solution would probably be to use a proper ES export for expect.

Link to repl or repo (highly encouraged)

repl.it link clicky clicky

envinfo

 npx envinfo --preset jest
npx: installed 1 in 4.202s

  System:
    OS: Linux 5.4 Debian GNU/Linux 9 (stretch) 9 (stretch)
    CPU: (4) x64 Intel(R) Xeon(R) CPU @ 2.30GHz
  Binaries:
    Node: 12.18.3 - /usr/local/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.6 - /usr/local/bin/npm
  npmPackages:
    jest: ^26.5.3 => 26.5.3 

 

[NaridaL]

Unfortunately, #10642 was closed by #11211, which only concerns the "using globals" case, not the issue described by #10642.

@SimenB I noted possible solutions both in the issue description an in #10642 (comment), but didn't get an answer. It would be appreciated if you could take another look.

[PaperStrike]

For anyone looking for a workaround, here's a dirty one:

// Use module augmentation on the internal 'expect/build/types'
declare module 'expect/build/types' {
  interface Matchers<R> {
    // ...
  }
}

[SimenB]

This will be supported in Jest 28 (currently available as jest@next), see example: https://github.com/facebook/jest/blob/eafc350f648ec75d7fb691b4ac45e02de8d68ada/examples/expect-extend/toBeWithinRange.ts#L49-L56

[github-actions]

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