[test-runner-mocha] implement snapshot testing

chai-dom-diff offers snapshot testing via

expect(element).shadowDom.to.equalSnapshot()

This fails in web test runner. Proximal cause is that window.__mocha_context__ is undefined. (this is probably a karma API?)

@LarsDenBakker recommended implementing snapshots directly with mocha, without reference to karma’s globals.

[LarsDenBakker]

@open-wc/semantic-dom-diff now also supports snapshot testing with WTR. Currently open-wc is in “prerelease” mode until the next version of lit is finalized. For now you can get the latest using the next tag for the testing package:

"@open-wc/testing": "next"

Note that snapshot testing is now async, so you need to await the assertion:

await expect(element).shadowDom.toEqualSnapshot();

[MaximePlancke]

Hi everyone,

Do you have any update on when the snapshot would be available with web test runner? I am having some issues with karma and @open-wc/testing (I have some commonJS packages and the preprocessors I tried to convert to esm does not seem to work with @open-wc/testing-karma).

My best bet was web test runner as it makes it very easy to convert CommonJS module with rollup but unfortunately my snapshot tests are not working.

Thank you 😃