Category: JavaScript Answers

To mock a dependency’s constructor with Jest, we can call jest.mock.

For instance, we write

import * as AWS from 'aws-sdk';

jest.mock('aws-sdk', () => {
  return {
    CloudWatch: jest.fn().mockImplementation(() => {
      return {}
    })
  }
});

test('AWS.CloudWatch is called', () => {
  new AWS.CloudWatch();
  expect(AWS.CloudWatch).toHaveBeenCalledTimes(1);
});

to mock the CloudWatch constructor with a function that returns an empty object.

We call jest.fn to return a mocked function and we mock the return value of the function with mockImplementation.

Then we can use the AWS.CloudWatch constructor to run the mocked version of the constructor.

We still have to import the real dependency with

import * as AWS from 'aws-sdk';

before we can mock AWS.CloudWatch.

Sometimes, we want to run Jest tests only for current folder.

In this article, we’ll look at how to run Jest tests only for current folder.

How to run Jest tests only for current folder?

To run Jest tests only for current folder, we can run jest with the path of the folder with the tests we want to run.

For instance, we run

npm test dev/app

to run the tests in the dev/app folder given that the test script runs jest.

We can also set the testPathPattern option to the folder with the tests we want to run.

For instance, we run

jest --watchAll --testPathPattern=dev/app

to run the tests in dev/app.

Conclusion

To run Jest tests only for current folder, we can run jest with the path of the folder with the tests we want to run.

Sometimes, we want to mock platform detection in Jest and React Native.

In this article, we’ll look at how to mock platform detection in Jest and React Native.

How to mock platform detection in Jest and React Native?

To mock platform detection in Jest and React Native, we can call jest.mock to mock the Platform module.

For instance, we write:

jest.mock('Platform', () => {
  const Platform = require.requireActual('Platform');
  Platform.OS = 'android';
  return Platform;
});

to call jest.mock with 'Platform and a function that return the Platform object.

Before we return it, we set the Platform.OS property to 'android' so Platform.OS would be 'android' in our tests.

Conclusion

To mock platform detection in Jest and React Native, we can call jest.mock to mock the Platform module.

To mock React custom hook returned value with Jest, we can call jest.mock.

For instance, we write:

jest.mock('module_name', () => ({
  useRect: () => [300, 200, jest.fn()]
}));

to call jest.mock with the module name and the function to mock the useRect hook with a function that returns the mocked values of the hook.

If we want to mock the hook once for a test, then use jest.spyOn with

import * as hooks from 'module_name';

it('a test', () => {
  jest.spyOn(hooks, 'useRect').mockImplementation(() => ([100, 200, jest.fn()]));
  // ...
});

to mock the useRect hook and return the mocked return values with mockImplementation.

We can just mock the return values since hooks are pure functions so they don’t commit side effects.