I have a very simple Typescript project with following structure

project/
├── build/
│   ├── index.d.ts
│   ├── index.js
│   └── index.js.map
├── docs/
│   └── README.md
├── source/
│   └── index.ts
├── tests/
│   └── index.test.ts
├── types/
│   └── global.d.ts
├── jest.config.json
├── package.json
└── tsconfig.json

I have a custom declaration file under /types directory. The content looks like this.

export {};

declare global {
  interface Error {
    deepClone(): this;
  }
}

I have updated my tsconfig.json to respect my type declaration.

{
  "compilerOptions": {
    "module": "ES2022",
    "target": "ES2022",
    "allowJs": true,
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true,
    "moduleResolution": "Bundler",
    "resolveJsonModule": true,
    "strict": true,
    "outDir": "./build",
    "sourceMap": true,
    "isolatedModules": false
  },
  "include": ["source/**/*.ts", "types/global.d.ts"],
  "exclude": ["node_modules", "tests", "build"]
}

I can perfectly build the project. But when i want to write tests for this JEST seems not beeing able to pick up the global.d.ts.

In my index.test.ts there is following line.

const deepCloneSpy = jest.spyOn(Error.prototype, "deepClone");

JEST now complains that the deepClone method is not on the Error.prototype even tough it should because of the custom type declaration.

When i copy the global.d.ts to the build/ directory it works. I am confused why JEST is not pickung up the global.d.ts because it’s specified in the include section of the tsconfig.json file. Why does it need to be in the build/ folder?