jsdom update 5 days ago breaks back compatability

🐛 Bug Report

jest.spyOn(localStorage, “setItem”); doesn’t work;

jest-environment-jsdom has caret dependency on jsdom -> 11.5.1, jsdom latest version is 11.12.0 and it includes localStorage and sessionStorage. Previously we could mock localStorage easily but now we don’t have that option and we need to use the default localStorage implementation. So our option is to spy on it which doesn’t work because it throws TypeError: object[methodName].mockImplementation is not a function

What I have discovered is that in ./node_modules/jsdom/lib/jsdom/living/generated/Storage.js the issue is with the set method of the Proxy

Currently the code below is on line 278.

if (ownDesc === undefined) {
          const parent = Reflect.getPrototypeOf(target);
          if (parent !== null) {
            return Reflect.set(parent, P, V, receiver);
          }
          ownDesc = { writable: true, enumerable: true, configurable: true, value: undefined };
        }

if we remove receiver from return Reflect.set(parent, P, V, receiver); we will be able to spy on it. But I guess that’s coming from webidl converter

To Reproduce

Steps to reproduce the behavior:

1. Install jest.
2. Write a simple class that leverage localStorage.setItem() or localStorage.getItem()
3. Try to spy on it -> jest.spyOn(localStorage, “setItem”); and it will throw an error

Expected behavior

The method should be available for spying.

Link to repl or repo (highly encouraged)

https://github.com/vlad0/jest-localstorage-issue

Run npx envinfo --preset jest

1. npm install
2. npm test Paste the results here:

> jest

 FAIL  ./service.spec.js
  Service
    ✕ Service set value (7ms)

  ● Service › Service set value

    TypeError: object[methodName].mockImplementation is not a function

       8 |
       9 |     it('Service set value', () => {
    > 10 |         jest.spyOn(localStorage, "setItem");
      11 |         service.setValue("hello", "world")
      12 |
      13 |         expect(localStorage.setItem).toHaveBeenCalled();

      at ModuleMockerClass.spyOn (node_modules/jest-mock/build/index.js:597:26)
      at Object.it (service.spec.js:10:14)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   0 total
Time:        0.785s, estimated 1s
Ran all test suites.
npm ERR! Test failed.  See above for more details.

[mr-wildcard]

We found out that localStorage from Jest is an instance of a class called Storage provided by jsdom. And using spyOn this way works :

jest.spyOn(Storage.prototype, 'setItem');

[omalraj]

jest.spyOn(window.localStorage.__proto__, 'setItem'); should work without any additional imports.