TypeScript file with an API call and tests with jest

api.test.ts

import {describe, expect, test} from '@jest/globals';
import * as API from './api';
import fetch from 'jest-mock-fetch';

describe('api module', () => {
  test('addProp should add bootcamp', async () => {
    /**
     * spying on the fetchUser method to check if it is called
     * by addProp
     * we're also mocking a return value from the fetchUser
     * method to check the value returned
     * from the addProp function at the end
     */
    jest.spyOn(API, 'fetchUser').mockReturnValueOnce(
      Promise.resolve({
        user: 'Ahsan'
      })
    )
    /**
     * Spying on the generateImages method to check if
     * it is called by addProp
     */
    jest.spyOn(API, 'generateImages')
    // store the value returned from addProp
    const returnedValue = await API.addProp()
    // check if fetchUser was called
    expect(API.fetchUser).toHaveBeenCalled()
    /**
     * check if the returned value from addProp contains bootcamp
     */
    expect(returnedValue.bootcamp).toBe('javascript')
    expect(API.generateImages).toHaveBeenCalled()
  })
  test('fetchUser should get data from the API and return it', (done) => {
    /**
     * Calling the actual method expecting correct data
     * This function calls a real `fetch` call. but we mock the
     * `fetch` using `jest-mock-fetch` at the top of this file.
    */
   API.fetchUser().then(data => {
     expect(data).toEqual({
       user: 'Ahsan'
      });
      done();
    })
    /**
     * Expecting the mock `fetch` to be called with the API
    */
   expect(fetch).toHaveBeenCalledWith("https://randomuser.me/api");
   /**
    * Here is where we're mocking the response from the mock fetch.
    */
   fetch.mockResponse({
     json: () => ({
       user: 'Ahsan'
     }) as object
   });
  });
});

api.ts

export const fetchUser = async () => {
  const resp = await fetch('https://randomuser.me/api')
  const data = await resp.json();
  return data;
}

export const generateImages = () => {
  // some code here
}

export  const addProp = async () => {
  const response = await fetchUser();
  response.bootcamp = 'javascript';
  generateImages()
  return response;
}