Warn against getter throwing exception since it is a side effect

[wolenetz]

Also includes some bikeshed indentation error fixes.

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Preview | Diff

[annevk]

To be clear, in general I agree that you probably do not want your getters to throw, but I think in practice all platform object getters can throw. For instance, if their this doesn't have the correct platform object brand.

cc @domenic

[annevk]

I'm not sure it's widely accepted that exceptions are side effects. E.g., I'm pretty sure typical side-effect-free functions are still allowed to do input validation.

I can kinda see how they can cause side effects in JavaScript, but only if you have listeners in place that end up modifying state in response to the exception.

[LeaVerou]

I think this is beside the point, because exceptions being side effects is not the reason getters should not throw. Getters should behave like regular JS properties, and these do not throw.
Also, typically, invalid state throws when set, it's not a good design to allow writes to go through only to realize you have invalid state when someone tries to read it.

[annevk]

But then explain why getters should not throw and don't state that exceptions are side effects. (Which I see you are doing below, so I'm not sure how my comment was off point.)

As to invalid state, properties might not apply to all states the object can be in. This is quite common with HTMLInputElement for instance. I'm not talking about the value of the property here.

[annevk]

I think it can still be reasonable if that only happens in a new state that was not accessible before the update, but this is an interesting observation.

[hober]

Given @annevk's comment, perhaps must should be watered down a bit?

[annevk]

(Sorry for not calling out these caveats earlier. I guess they didn't really occur to me when you filed the issue.)

[cynthia]

Suggested change

	Updating existing APIs to include getters that throw exception must be avoided.
	Updating existing APIs to include getters that throw an exception must be avoided.

[LeaVerou]

Suggested change

	Exceptions are side effects.
	Getters should [behave like regular data properties](#attributes-like-data), and regular data properties do not throw exceptions when read. Furthermore, invalid state should generally be avoided by rejecting *writes*, not when data is *read*.

[LeaVerou]

Suggested change

	Getters should [behave like regular data properties](#attributes-like-data), and regular data properties do not throw exceptions when read. Furthermore, invalid state should generally be avoided by rejecting *writes*, not when data is *read*.
	Getters and setters should [behave like regular data properties](#attributes-like-data),
	and regular data properties do not throw exceptions when read or set.
	Invalid state should generally be avoided by rejecting *writes*, not when data is *read*,
	and if setting data could throw exceptions, a method is generally preferable over a setter.

[torgo]

We discussed in today's call and we agreed to merge without Lea's proposed change for now.