How can jest's expect.extend() be applied to a specific test file using TypeScript?

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To apply jest's expect.extend() method to a specific test file using TypeScript, you can follow these steps:

1. Create a file called setup-tests.ts in the same directory as your test file.

2. Inside setup-tests.ts, import @jest/globals and the TypeScript types for ExpectExtendMap and Matchers.

   import '@jest/globals';
   import { ExpectExtendMap, Matchers } from '@jest/types';
   

3. Define your custom matcher by extending Matchers and exporting it.

   export interface CustomMatchers extends Matchers<unknown> {
    toBeValidEmail(): void;
   }
   
   export const customMatchers: ExpectExtendMap = {
    toBeValidEmail(received) {
      // validation logic here
      expect(received).toBeTruthy();
    },
   };
   

4. In your test file, import setup-tests.ts and call expect.extend() with your custom matcher.

   import './setup-tests';
   
   it('should validate email addresses', () => {
    expect('example@domain.com').toBeValidEmail();
   });
   

5. Ensure that your test file is being transpiled by TypeScript by adding the appropriate configuration in tsconfig.json.

   {
    "compilerOptions": {
      "target": "esnext",
      "module": "commonjs",
      "esModuleInterop": true,
      "outDir": "dist",
      "types": ["jest"],
      "typeRoots": ["./node_modules/@types", "./src/types"],
      "strict": true
    },
    "include": ["src/**/*", "test/**/*"],
    "exclude": ["node_modules", "**/*.spec.*", "**/__tests__/*"]
   }
   

That's it! You can now use your custom matcher in your test file.