Extension of a third-party module that is globally open

Question

I am trying to add a custom match to Jest in Typescript. This works fine, but I can't get Typescript to recognize advanced Matchers.

myMatcher.ts

export default function myMatcher (this: jest.MatcherUtils, received: any, expected: any): { pass: boolean; message (): string; } {
  const pass = received === expected;
  return {
    pass: pass,
    message: () => `expected ${pass ? '!' : '='}==`,
  }
}

myMatcher.d.ts

declare namespace jest {
  interface Matchers {
    myMatcher (expected: any): boolean;
  }
}

someTest.ts

import myMatcher from './myMatcher';

expect.extend({
  myMatcher,
})

it('should work', () => {
  expect('str').myMatcher('str');
})

tsconfig.json

{
  "compilerOptions": {
    "outDir": "./dist/",
    "moduleResolution": "node",
    "module": "es6",
    "target": "es5",
    "lib": [
      "es7",
      "dom"
    ]
  },
  "types": [
    "jest"
  ],
  "include": [
    "src/**/*"
  ],
  "exclude": [
    "node_modules",
    "dist",
    "doc",
    "**/__mocks__/*",
    "**/__tests__/*"
  ]
}

In someTests.ts I get an error

error TS2339: Property 'myMatcher' does not exist on type 'Matchers'

I read the Microsoft documentation several times, but I can’t figure out how to change the namespace with globally accessible types (not exported).

Putting it in index.d.ts from jest works fine, but is not a good solution for a rapidly changing code base and classes that are extensible by several parties.

Answer 1

Ok, there are a few questions here

(.ts .tsx) (.d.ts) , , .

, , , jest. , TypeScript . , , .ts

myMatcher.ts

// use declare global within a module to introduce or augment a global declaration.
declare global {
  namespace jest {
    interface Matchers {
      myMatcher: typeof myMatcher;
    }
  }
}
export default function myMatcher<T>(this: jest.MatcherUtils, received: T, expected: T) {
  const pass = received === expected;
  return {
    pass,
    message: () => 'expected ${pass ? '!' : '='}=='
  };
}

, , . ,

myMatcher.ts

// ensure this is parsed as a module.
export {};

declare global {
  namespace jest {
    interface Matchers {
      myMatcher: typeof myMatcher;
    }
  }
}
function myMatcher<T>(this: jest.MatcherUtils, received: T, expected: T) {
  const pass = received === expected;
  return {
    pass,
    message: () => 'expected ${pass ? '!' : '='}=='
  };
}

expect.extend({
  myMatcher
});

someTest.ts

import './myMatcher';

it('should work', () => {
  expect('str').myMatcher('str');
});

Answer 2

:

customMatchers.ts

declare global {
    namespace jest {
        interface Matchers<R> {
            // add any of your custom matchers here
            toBeDivisibleBy: (argument: number) => {};
        }
    }
}

// this will extend the expect with a custom matcher
expect.extend({
    toBeDivisibleBy(received: number, argument: number) {
        const pass = received % argument === 0;
        if (pass) {
            return {
                message: () => 'expected ${received} not to be divisible by ${argument}',
                pass: true
            };
        } else {
            return {
                message: () => 'expected ${received} to be divisible by ${argument}',
                pass: false
            };
        }
    }
});

my.spec.ts

import "path/to/customMatchers";

test('even and odd numbers', () => {
   expect(100).toBeDivisibleBy(2);
   expect(101).not.toBeDivisibleBy(2);
});