How to create a mock object from an interface in Typescript with Jest?

Question

I've the following code:

import { CreatePageArgs } from 'gatsby';
import { onCreatePage } from './onCreatePage';
jest.mock('gatsby');

describe('on create page', () => {
  test('onCreatePage', () => {
    const pageArgs = {} as CreatePageArgs;
    pageArgs.page.internal.type = 'AnotherType';
    onCreatePage(pageArgs);
  });
});

During the execution I get the following error:

 FAIL  src/onCreatePage.test.ts
  ● on create page › onCreatePage

    TypeError: Cannot read property 'internal' of undefined

       6 |   test('onCreatePage', () => {
       7 |     const pageArgs = {} as CreatePageArgs;
    >  8 |     pageArgs.page.internal.type = 'AnotherType';
         |                   ^
       9 |     onCreatePage(pageArgs);
      10 |   });
      11 | });

      at Object.<anonymous> (src/onCreatePage.test.ts:8:19)

How can I deep instantiate a object from an interface in TypeScript with Jest?

pascalpuetz · Accepted Answer · 2020-01-03 19:15:33Z

1

You can't. Interfaces are a concept in TypeScript that only exist during compile time. They do not exist during runtime, hence Jest has no way of using it to construct an object.

answered Jan 3, 2020 at 19:15

pascalpuetz

5,4281 gold badge15 silver badges26 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Robin Over a year ago

Hmm.. I've seen this: github.com/marchaos/jest-mock-extended So it seems to be possible. Why isn't it integrated into jest?

Collectives™ on Stack Overflow

How to create a mock object from an interface in Typescript with Jest?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related