fix: don't throw error for tagName getter in Jest

[nolanlawson]

Details

Fixes salesforce/sfdx-lwc-jest#299

I believe that, long-term, we should do #3245 and eliminate functional differences between dev mode and prod mode. However, that is a larger effort, whereas this is a quick fix.

It is currently not possible to write a test for this, because we don't run any engine-dom or engine-core tests in Jest.

Does this pull request introduce a breaking change?

• ✅ No, it does not introduce a breaking change.

Does this pull request introduce an observable change?

• ✅ No, it does not introduce an observable change.

[jye-sf]

We have a karma test for this at https://github.com/salesforce/lwc/tree/master/packages/%40lwc/integration-karma/test/component/LightningElement.tagName. May need to update that, and you could test this new functionality there.

[nolanlawson]

@jye-sf The karma tests don't run with process.env.NODE_ENV === 'test' enabled. That is only used for Jest.

[nolanlawson]

/nucleus test

[ravijayaramappa]

I tried this change with the reported test case, the new assertion message is

    expect(jest.fn()).not.toBeCalled()

    Expected number of calls: 0
    Received number of calls: 1

    1: {Symbol(@@lockerLiveValue): undefined}

      10 |         document.body.appendChild(element);
      11 |         debugger;
    > 12 |         expect(foo).not.toBeCalled();
         |                         ^
      13 |     });
      14 | });

      at Object.toBeCalled (force-app/main/default/lwc/myComponent/__tests__/index.spec.js:12:25)

The error message isn't much useful to determine that the value is an LWC Component. Jest serializes it like an Object and prints out the enumerable properties. This is acceptable.
However, if we want to make it a bit better, IMO the solution is to provide a serializer for LightningElement component instance via @lwc/jest-serializer and have it call the LightningElement.toString() method

The background is that jest tries to detect the value type. For DOMElement, it uses the tagName property to test if the value is a DOMElement. Hence the error reported by the user. The code is in pretty-format package.

[nolanlawson]

However, if we want to make it a bit better, IMO the solution is to provide a serializer for LightningElement component instance via @lwc/jest-serializer and have it call the LightningElement.toString() method

This makes sense; we can open an issue on lwc-test for that.

AIUI, if we did this, then we would not need to add any code to LWC OSS to do special handling for Jest. So yeah, that makes sense to me as an alternative.

[nolanlawson]

salesforce/lwc-test#168