mocking-functions.ts

// under-test.ts

import { myFunc } from './service';

export class UnderTest {
  display() {
    const val = myFunc('abc');
    console.log('display');
    return val;
  }
}


// service.ts

export const myFunc = (value) => {
  console.log('#myFunc value: ', value);
  return 'val from myFunc';
};


// undert-test.spec.ts

import { UnderTest } from './under-test';
import * as service from './service';

describe('UnderTest', () => {
  describe('#display', () => {
    it('returns value from my func', () => {
      spyOn(service, 'myFunc').and.returnValue('my-value');
      const obj = new UnderTest();

      expect(obj.display()).toEqual('my-value');
    });

    it('calls my func', () => {
      spyOn(service, 'myFunc').and.returnValue('my-value');

      const obj = new UnderTest();
      obj.display();

      expect(service.myFunc).toHaveBeenCalledWith('abc');
    });
  });
});