fix(ssr): only default-exported classes are LWC components @W-17312358@

[wjhsf]

Details

Fixes #4938.

Does this pull request introduce a breaking change?

• 😮‍💨 No, it does not introduce a breaking change.
• 💔 Yes, it does introduce a breaking change.

Does this pull request introduce an observable change?

• 🤞 No, it does not introduce an observable change.
• 🔬 Yes, it does include an observable change.

GUS work item

[nolanlawson]

There is actually a lot of logic that the babel-plugin-component currently does to try to distinguish LightningElement classes from non-LE classes:

lwc/packages/@lwc/babel-plugin-component/src/component.ts

Lines 38 to 45 in fb163f7

	function needsComponentRegistration(path: DeclarationPath) {
	return (
	(path.isIdentifier() && path.node.name !== 'undefined' && path.node.name !== 'null') ||
	path.isCallExpression() ||
	path.isClassDeclaration() ||
	path.isConditionalExpression()
	);
	}

lwc/packages/@lwc/babel-plugin-component/src/decorators/index.ts

Lines 289 to 295 in fb163f7

	if (
	node.superClass === null &&
	isAPIFeatureEnabled(
	APIFeature.SKIP_UNNECESSARY_REGISTER_DECORATORS,
	getAPIVersionFromNumber(state.opts.apiVersion)
	)
	) {

The first controls registerComponent – if a component is not registered, then we'll throw an exception at runtime.

The second controls whether decorators are transformed.

I guess at the end of the day we only have to be 1-to-1 compatible if 1) a compiler error is not going to be thrown somewhere, and 2) the ssr-compiler is stricter than the pre-existing toolchain. (#1 is because, on core, components will be compiled both for SSR and CSR, so if CSR throws then we are fine. Dangling @apis on non-LE classes will throw eventually because decorators are not supported natively in Babel yet.)

So I guess I am saying this is fine, but we will probably keep discovering edge cases as we go.

[wjhsf]

TODO: What happens if this mixin has decorated props?

[cardoso]

I actually do that in production 😆

[nolanlawson]

• [SSR] Add test for decorators within a mixin class #4962