[Bug]: inital window.guardian can be incorrectly typed due to serialization

[Jakeii]

The default value of window.guardian that's injected into the page goes through JSON.strigify, this strips any incompatible keyvalues - namely functions and keys with undefined values.

This is somewhat unexpected! There are a few functions stubbed in createGuardian, that do not actually get on to the page, until they're later added by client side code.

this

{
...
		modules: {
			// This is a stub for the sentry module, which is later initialised on the client with the `reportError` function.
			sentry: {
				reportError: () => {},
			},
			// This is a stub for the abTests module, which is later initialised on the client with the `getParticipations`, `isUserInTest` and `isUserInTestGroup` functions.
			abTests: {
				getParticipations: () => ({}),
				isUserInTest: () => false,
				isUserInTestGroup: () => false,
			},
		},
}

becomes this when stringified:

{"modules":{"sentry":{},"abTests":{}}}`

This led to a hard to track bug in commercial where we expected window.guardian.modules.abTests to be

{
  getParticipations: () => ({}),
  isUserInTest: () => ({}),
  isUserInTestGroup: () => ({}),
} || undefined

but {} wasn't expected!

type-fest has some types to check for this sort of thing, perhaps they could be incorporated to ensure that the returntype of createGuardian is safely serializable?

[SiAdcock]

Interesting @Jakeii. I've got a couple of questions, based mostly on ignorance

1. If some external scripts already rely on the abTests object, how would we preserve this capability in a world in which the values need to be serialisable?

2. What would you expect the outcome to be if a non-serialisable type is used?

• build fails?
• compilation fails?
• console error message in dev?
• ...?