Define 'ts-jest' property on ConfigGlobals interface in @jest/types via declaration merging

[G-Rath]

🚀 Feature Proposal

A few months back I converted the jest ConfigGlobal type from a type to an interface to allow declaration merging, semi-specifically for ts-jest:

// ts-jest/index.d.ts
declare module '@jest/types' {
  namespace Config {
    interface ConfigGlobals {
      'ts-jest': TsJestGlobalOptions;
    }
  }
}

// jest.config.js
/** @type {import('@jest/types').Config.InitialOptions} */
const config= {
  globals: {
    'ts-jest': {
      isolatedModules: true,
      myProperty: 1 // error, "myProperty" is not defined!
    }
  }
}

module.exports = config;

....and then I never got around to opening this issue or making a PR 😅

Motivation

This pattern should help reduce misconfiguration issues, such as #1564 :)

Example

The caveat which would be useful to document is that you need to give TypeScript a reason to look in ts-jest/index.d.ts from jest.config.js. The easiest way of doing this is to import ts-jest either via jsdoc (/** @typedef {import('ts-jest')} */) or at runtime (require('ts-jest') or import 'ts-jest').

[ahnpnl]

This looks really nice, it helps #925 too. PR is welcome !

[ahnpnl]

Close via #1592