Jasmine Spy on Properties on Mock Objects

Jasmine Spy on Properties on Mock Objects

Takahiro Iwasa
(岩佐 孝浩)
Takahiro Iwasa (岩佐 孝浩)
3 min read
Angular JavaScript

When attempting to spy on a property of an already mocked object, developers should see the following error.

Error: <spyOnProperty> : currentUser#get has already been spied upon
Usage: spyOnProperty(<object>, <propName>, [accessType])

This error is avoidable with Object.getOwnPropertyDescriptor.

Situation

When implementing the testing code like the following, the error was raised.

import { HttpClientTestingModule } from '@angular/common/http/testing';
import { TestBed } from '@angular/core/testing';
import { AuthService } from '@core/services/auth.service';

describe('AuthService', () => {

  let service: jasmine.SpyObj<AuthService>;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [HttpClientTestingModule],
      providers: [
        {provide: AuthService, useValue: jasmine.createSpyObj('AuthService', [], ['currentUser'])},
      ],
    });

    service = TestBed.inject(AuthService) as jasmine.SpyObj<AuthService>;
  });

  it('should get the currentUser', () => {
    spyOnProperty(service, 'currentUser').and.returnValue({id: 1, name: 'Hello World'});
    // Testing code here...
    expect(service.currentUser).toEqual({id: 1, name: 'Hello World'});
  });

});

Solution

The official tutorial says that we can use Object.getOwnPropertyDescriptor.

You can create a spy object with several properties on it quickly by passing an array or hash of properties as a third argument to createSpyObj. In this case you won’t have a reference to the created spies, so if you need to change their spy strategies later, you will have to use the Object.getOwnPropertyDescriptor approach.

This was also referred to in a Stack Overflow question.

Helper Functions

I wrote helper functions and placed it in src/app/tests/helper.ts.

/* eslint-disable-next-line arrow-body-style */
export const spyGetter = <T, K extends keyof T>(target: jasmine.SpyObj<T>, key: K): jasmine.Spy => {
  return Object.getOwnPropertyDescriptor(target, key)?.get as jasmine.Spy;
};

/* eslint-disable-next-line arrow-body-style */
export const spySetter = <T, K extends keyof T>(target: jasmine.SpyObj<T>, key: K): jasmine.Spy => {
  return Object.getOwnPropertyDescriptor(target, key)?.set as jasmine.Spy;
};

Updated Testing Code

After updating my testing code, it ran successfully.

import { HttpClientTestingModule } from '@angular/common/http/testing';
import { TestBed } from '@angular/core/testing';
import { AuthService } from '@core/services/auth.service';
import { spyGetter } from '@tests/helper';

describe('AuthService', () => {

  let service: jasmine.SpyObj<AuthService>;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [HttpClientTestingModule],
      providers: [
        {provide: AuthService, useValue: jasmine.createSpyObj('AuthService', [], ['currentUser'])},
      ],
    });

    service = TestBed.inject(AuthService) as jasmine.SpyObj<AuthService>;
  });

  it('should get the currentUser', () => {
    spyGetter(service, 'currentUser').and.returnValue({id: 1, name: 'Hello World'});
    // Testing code here...
    expect(service.currentUser).toEqual({id: 1, name: 'Hello World'});
  });

});

The difference is as follows:

--- 	Sun Sep 26 11:05:21 2021 UTC
+++ 	Sun Sep 26 11:05:21 2021 UTC
@@ -1,6 +1,7 @@
 import { HttpClientTestingModule } from '@angular/common/http/testing';
 import { TestBed } from '@angular/core/testing';
 import { AuthService } from '@core/services/auth.service';
+import { spyGetter } from '@tests/helper';

 describe('AuthService', () => {

@@ -18,7 +19,7 @@
   });

   it('should get the currentUser', () => {
-    spyOnProperty(service, 'currentUser').and.returnValue({id: 1, name: 'Hello World'});
+    spyGetter(service, 'currentUser').and.returnValue({id: 1, name: 'Hello World'});
     // Testing code here...
     expect(service.currentUser).toEqual({id: 1, name: 'Hello World'});
   });
Takahiro Iwasa
(岩佐 孝浩)

Takahiro Iwasa (岩佐 孝浩)

Software Developer at iret, Inc.
Architecting and developing cloud native applications mainly with AWS. Japan AWS Top Engineers 2020-2023