Prevent the necessity of typecasting in mocked TypeScript imports

Question

Prevent the necessity of typecasting in mocked TypeScript imports

When I import a mocked function, Typescript doesn't recognize that Jest changes the import to a mock. As a result, I have to cast the imported function in order to utilize mock methods such as mockReturnValue.

jest.mock('../myImport');
import { thisFunctionIsMocked } from '../myImport'

/* ... */

(<Mock<any>>thisFunctionIsMocked).mockReturnValue(42);

If I don't cast the import, Typescript gives an error claiming that the mock function methods do not exist. Is there a more efficient way to handle this situation?

typescript jestjs

Answer 1

Answer №1

I've been searching for a solution to this same issue. It appears there isn't much information available on how to tackle it, but there are a few alternatives to consider.

1) Try using a slightly different syntax from what you're currently using:

(thisFunctionIsMocked as jest.Mock).mockReturnValue(42);

2) Another option is to use require and cast the function there:

const thisFunctionIsMocked = require('../myImport') as jest.Mock;

Update:

The ts-jest now provides a mocked helper that can be utilized. This ensures that the type signature of the original source remains intact even when referencing mocked imports:

import { mocked } from 'ts-jest/utils';

test('direct', () => {
  foo.name();
  expect(mocked(foo.name).mock.calls).toHaveLength(1);
});

Answer 2

I've been searching for a solution to this same issue. It appears there isn't much information available on how to tackle it, but there are a few alternatives to consider.

1) Try using a slightly different syntax from what you're currently using:

(thisFunctionIsMocked as jest.Mock).mockReturnValue(42);

2) Another option is to use require and cast the function there:

const thisFunctionIsMocked = require('../myImport') as jest.Mock;

Update:

The ts-jest now provides a mocked helper that can be utilized. This ensures that the type signature of the original source remains intact even when referencing mocked imports:

import { mocked } from 'ts-jest/utils';

test('direct', () => {
  foo.name();
  expect(mocked(foo.name).mock.calls).toHaveLength(1);
});

Answer 3

Answer №2

Inspired by @lobati's solution, here is a modified approach for handling mock modules with multiple exports:

import * as myModuleMock from 'path/to/my/module';
const { someExport, someOtherExport } = 
    require('path/to/my/__mocks__/module') as typeof myModuleMock;

// The types of `someExport` and `someOtherExport` will be inferred correctly 
/// based on the type information provided in the actual mock

By utilizing this method, there is no need to duplicate type information from the mock module or manage each export individually.

Answer 4

Inspired by @lobati's solution, here is a modified approach for handling mock modules with multiple exports:

import * as myModuleMock from 'path/to/my/module';
const { someExport, someOtherExport } = 
    require('path/to/my/__mocks__/module') as typeof myModuleMock;

// The types of `someExport` and `someOtherExport` will be inferred correctly 
/// based on the type information provided in the actual mock

By utilizing this method, there is no need to duplicate type information from the mock module or manage each export individually.

Answer 5

Answer №3

In order to easily and cleanly apply mocks until Jest can intercept ES6 modules, I found this method to be the most suitable for my specific situation.

jest.mock('./some/resource', () => ({
  thisFunctionIsMocked: jest.fn()
})

const { thisFunctionIsMocked } = require('./some/resource');

test('works', () => {
  thisFunctionIsMocked.mockReturnValue('cool result');
  expect(thisFunctionIsMocked()).toEqual('cool result');
});

It's important to point out the usage of require instead of import.

If you are incorporating module mocks, make sure to export them using CommonJS syntax:

module.exports = {
  thisFunctionIsMocked: jest.fn()
};

Once again, this is due to Jest not being able to intercept ES6 modules as of now.

Answer 6

In order to easily and cleanly apply mocks until Jest can intercept ES6 modules, I found this method to be the most suitable for my specific situation.

jest.mock('./some/resource', () => ({
  thisFunctionIsMocked: jest.fn()
})

const { thisFunctionIsMocked } = require('./some/resource');

test('works', () => {
  thisFunctionIsMocked.mockReturnValue('cool result');
  expect(thisFunctionIsMocked()).toEqual('cool result');
});

It's important to point out the usage of require instead of import.

If you are incorporating module mocks, make sure to export them using CommonJS syntax:

module.exports = {
  thisFunctionIsMocked: jest.fn()
};

Once again, this is due to Jest not being able to intercept ES6 modules as of now.

Answer 7

Answer №4

When it comes to module augmentation, there are certain limitations you need to be aware of. For instance, if you export a function in a certain way, you may not be able to modify existing types.

export function thisFunctionIsMocked() { .... }

This is because types can only be added to, not altered. However, if you export your function like this:

export interface MyMockedFunction {
    (): void;
}

export const thisFunctionIsMocked = function() { ... } as MyMockedFunction;

You can then proceed to augment it accordingly:

jest.mock('../myImport');
import { thisFunctionIsMocked } from '../myImport'

declare module "../myImport" {
    interface MyMockedFunction {
        mockReturnValue(value: number): any;
    }
}

thisFunctionIsMocked.mockReturnValue(42);

Answer 8

When it comes to module augmentation, there are certain limitations you need to be aware of. For instance, if you export a function in a certain way, you may not be able to modify existing types.

export function thisFunctionIsMocked() { .... }

This is because types can only be added to, not altered. However, if you export your function like this:

export interface MyMockedFunction {
    (): void;
}

export const thisFunctionIsMocked = function() { ... } as MyMockedFunction;

You can then proceed to augment it accordingly:

jest.mock('../myImport');
import { thisFunctionIsMocked } from '../myImport'

declare module "../myImport" {
    interface MyMockedFunction {
        mockReturnValue(value: number): any;
    }
}

thisFunctionIsMocked.mockReturnValue(42);

Prevent the necessity of typecasting in mocked TypeScript imports

Answer №1

Answer №2

Answer №3

Answer №4

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