Jest mock jsonwebtoken sign : NestJs

I’m trying to write unit test for my authenticateUser method.In side that method i’m generating private key signed jwt token and return to the user.But my signToken method seem not mocked properly.it return Promise {}.

userService.ts

  async authenticateUser(authDto: AuthDTO): Promise<AuthResponse> {
  
  const formatedMobile = this.commonHelper.formatMobile(
        decodedIdToken.Mobile,
      );
      
    const userData = await this.userRepository.findOne({
        msisdn: formatedMobile,
      });
  if (!userData) {
        //save user info
        const user = new UserEntity();
        user.firstName = decodedIdToken['first_name'];
        user.lastName = decodedIdToken['last_name'];
        user.msisdn = formatedMobile;
        await this.userRepository.save(user);
      }
    const jwtPayload={createdTime: moment(),msisdn: formatedMobile}
    
    const jwtToken = this.commonHelper.signToken(jwtPayload);
    
     return new AuthResponse(
        HttpStatus.OK,
        STRING.SUCCESS,
        `${STRING.USER} ${STRING.AUTHENTICATE} ${STRING.SUCCESS}`,
        {jwtToken},
      );
  }

userService.specs.ts

  it('authenticateUser should return success response (User Exist)', async () => {
    const mockJWTToken = {
    jwtToken:"eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJleHAiOjE2NDgyNjIzMDYsImRhdGEiOnsiY3JlY"
    }
    const mockAuthBody = { authCode: 'c79da50f-4eb1',platform:'MOBILE' };
    const mockSuccessResponse = new AuthResponse(
      HttpStatus.OK,
      STRING.SUCCESS,
      `${STRING.USER} ${STRING.AUTHENTICATE} ${STRING.SUCCESS}`,
      mockJWTToken,
    );

    jest.spyOn(commonHelper, 'formatMobile');
    jest.spyOn(userRepository, 'findOne').mockResolvedValueOnce(userEntity);
    jest.spyOn(commonHelper, 'signToken').mockResolvedValueOnce(mockJWTToken);

    expect(await userService.authenticateUser(mockAuthBody)).toEqual(
      mockSuccessResponse,
    );
  });