CODEWITHSUNDEEP
javascriptreact-nativejestjs
bsky
bsky

Reputation: 20222

Mocked function is `undefined` but unit test doesn't fail

In my unit test suite, I have the following mock:

  beforeEach(() => {
        NativeModules.MyModule = {
            myMethod: jest.fn()
        };
     })

And this unit test that uses it:

 it('has some functionality', () => {
    console.log(JSON.stringify(NativeModules.MyModule.myMethod));
    expect(NativeModules.MyModule.myMethod).toHaveBeenCalledTimes(1);
});

The console.log function prints undefined but the test passes.

However, if I add this line:

expect(undefined).toHaveBeenCalledTimes(1);

The test will fail with this message:

expect(jest.fn())[.not].toHaveBeenCalledTimes()

jest.fn() value must be a mock function or spy.
Received: undefined

So how can the unit test pass if NativeModules.MyModule.myMethod is undefined?

Upvotes: 1

Views: 436

Answers (1)

Craig Ayre
Craig Ayre

Reputation: 1173

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify

If undefined, a function, or a symbol is encountered during conversion it is either omitted (when it is found in an object) or censored to null (when it is found in an array). JSON.stringify can also just return undefined when passing in "pure" values like JSON.stringify(function(){}) or JSON.stringify(undefined).

If you log the mocked function directly (console.log(NativeModules.MyModule.myMethod) instead of logging console.log(JSON.stringify(NativeModules.MyModule.myMethod)) you should see the output you expect.

For example: 

console.log(() => {})
console.log(JSON.stringify(() => {}))

Upvotes: 2

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