Jest returning error when calling mockReturnValueOnce with jest.spyOn()

Question

jest --passWithNoTests --silent --noStackTrace --runInBand --watch -c jest-unit-config.js

Project repo

Error in jest.spyOn(bcrypt, 'hash').mockRejectedValue(new Error('Async error message'))

error TS2345: Argument of type 'Error' is not assignable to parameter of type 'never'.
31 jest.spyOn(bcrypt, 'hash').mockRejectedValue(new Error('Async error message'))

    import bcrypt from 'bcrypt'
    import { BcryptAdapter } from './bcrypt-adapter'

    jest.mock('bcrypt', () => ({
      async hash (): Promise<string> {
        return 'hash'
      }
    }))
    const salt = 12
    const makeSut = (): BcryptAdapter => {
      return new BcryptAdapter(salt)
    }
    
    describe('Bcrypt Adapter', () => `{`
    
      test('Should throw if bcrypt throws', async () => {
        const sut = makeSut()
        jest.spyOn(bcrypt, 'hash').mockRejectedValue(new Error('Async error message'))
        const promise = sut.encrypt('any_value')
        await expect(promise).rejects.toThrow()
      })
    })

If I use .mockImplementantion instead of .mockRejectedValue, another error occurs:

  test('Should throw if bcrypt throws', async () => {
    const sut = makeSut()
    jest.spyOn(bcrypt, 'hash').mockImplementation((): Promise<any> => { Promise.reject(new Error()) })
    const promise = sut.encrypt('any_value')
    await expect(promise).rejects.toThrow()
  })

error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value. 31 jest.spyOn(bcrypt, 'hash').mockImplementation((): Promise => { Promise.reject(new Error()) })

BcryptAdapter.encrypt

  async encrypt (value: string): Promise<string> {
    const hash = await bcrypt.hash(value, this.salt)
    return hash
  }

Package version:

    "@types/jest": "^27.5.0",
    "jest": "^28.1.0",
    "ts-jest": "^28.0.2",
    "typescript": "^4.6.4"

node v16.14.0

I know there is a workaround using .mockImplementation((): never => {throw new Error()}) but the question here is why the first two approaches didn't work.

Answer 1

Could you help remove this code

jest.mock('bcrypt', () => ({
    async hash (): Promise<string> {
       return 'hash'
    }
}))

because you use jest.spyOn to mock it return Error instead of string. Also I think we dont need jest.mock when using jest.spyOn here

Answer 2

You need a Mac device to run on iOS. By selecting a simulator, you can easily run from vscode/android studio/xcode. Also you don't need to change any code but if you want to show different style in different platform, you need to change those code accordingly.