Extending third party module that is globally exposed

Question

I am trying to add a custom matcher to Jest in Typescript. This works fine, but I can\'t get Typescript to recognize the extended Matchers.

myMatcher.ts

Answer 1

A simple way is:

customMatchers.ts

declare global {
    namespace jest {
        interface Matchers {
            // 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);
});