check if data is serializable

[Rich-Harris]

This throws an error if a load function in +page.server.js or +layout.server.js returns data that can't be serialized as JSON, so things like /hello/ don't become {}.

Supersedes #5635.

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[gtm-nayan]

This wouldn't work for custom classes. If they pass through they'll fail instanceof checks and their methods aren't iterated over by either Object.keys or for..in.

[Rich-Harris]

luckily i know where i can find a more robust check!

Suggested change

	const tag = Object.prototype.toString.call(value);
	if (tag === '[object Object]') {
	// This simple check might potentially run into some weird edge cases
	// Refer to https://github.com/lodash/lodash/blob/2da024c3b4f9947a48517639de7560457cd4ec6c/isPlainObject.js?rgh-link-date=2022-07-20T12%3A48%3A07Z#L30
	// if that ever happens
	if (Object.getPrototypeOf(value) === Object.prototype) {

[gtm-nayan]

That one is essentially "if it's not a freshly constructed plain object then no bueno",
I think it'll miss Object.defineProperties and other stuff, but if people are going to those lengths for the returned object, I wouldn't want to accomodate that.

[gtm-nayan]

Also, are we going to need these checks if #6008 is implemented?

[Rich-Harris]

No, in that case we'd have to rely on devalue throwing an error if stuff isn't serializable. Though it would be preferable to serialize the entire array in one go, which would make it tricker to track down the error... I haven't really thought about the best way to do that yet, but in any case #6008 probably won't happen for a while (feels like a post-1.0 thing)

[ghost]

ObjectIds get blocked despite passing a JSON.stringify({ a: new ObjectId() }) and leading to a string fully reversible back to an ObjectId. Is this intended?