How to add types for jest test with describe.each?

Question

const has = (object: Record<string, unknown>, key: string) => {
    return object != null && hasOwnProperty.call(object, key)
};

has.test.ts

describe('has', () => {
    const obj = {
        name: 'name',
        1: 1,
        false: false,
        undefined: undefined
    };
    describe.each([
        ['name', true],
        [1, true],
        [false, true],
        [undefined, true],
        ['no-such-key', false]
    ])('when key = %s', (key, expected) => {
        it(`should return ${expected}`, () => {
            expect(has(obj, key)).toBe(expected);
        });
    });
});

Does anyone have experience adding types for jest tests? I am using describe.each to loop over datasets. Though I am able to run the tests successfully, I want to fix that typing issue. Can someone help me?

Answer 1

It seems that you don't have latest version of types for jest, try to update package @types/jest to latest version (it contains type definition for Each interface).

If that's not possible for some reasons you can always "extend" the types yourself using typescript feature called declaration-merging:

// jest.d.ts file

declare namespace jest {

  interface Each {
    // Exclusively arrays.
    <T extends any[]>(cases: ReadonlyArray<T>): (name: string, fn: (...args: T) => any, timeout?: number) => void;
    // Not arrays.
    <T>(cases: ReadonlyArray<T>): (name: string, fn: (...args: T[]) => any, timeout?: number) => void;
    (cases: ReadonlyArray<ReadonlyArray<any>>): (
        name: string,
        fn: (...args: any[]) => any,
        timeout?: number
    ) => void;
    (strings: TemplateStringsArray, ...placeholders: any[]): (
        name: string,
        fn: (arg: any) => any,
        timeout?: number
    ) => void;
  }

  interface Describe {
    each: Each
  }
}

You might need also to specify typeRoots configuration option so that typescript can pick up your custom types

Upd: Sorry, I just noticed that your issue is not in absence of Each interface but in incorrect type. It seems that in your case typescript cannot infer the type correctly so you might want to specify generic type explicitly, e.g.:

type TestTuple = [string | number | boolean, boolean];

describe.each<TestTuple>([
  ['name', true],
  [1, true],
  [false, true],
  [undefined, true],
  ['no-such-key', false]
])('when key = %s', (a, b) => {
    // do your stuff
});