esm: add support for JSON import assertion

[aduh95]

Import assertion support has landed behind a flag in V8 for quite some time, this PR enables it by default in Node.js. This PR allows folks to write JS code that uses import assertions syntax. Only type: “json” is supported for now, although JSON modules support in Node.js is still experimental and behind --experimental-json-modules CLI flag.
This PR also disables importing of JSON modules without using an assertion, by popular demand. Support for those can be added using loader hooks.

Refs: #37375 (comment)
Alternative to #39921

/cc @nodejs/modules @nodejs/loaders

[ljharb]

Is there no way we could add a separate flag that allows assertionless JSON import?

[GeoffreyBooth]

@aduh95 Thank you for doing this, I really appreciate it. I don’t have any objections with your intent of this PR, so I’m happy for it to land once we can work through some technical discussions. I appreciate your patience.

In the meantime, can we close the other versions of this so that there’s no confusion? #39921 and #40210, are there any others? So that in the end it’s just the PR that adds support for assertions (this PR) and the PR that unflags JSON modules (#37375).

Also #40249 feels pretty interdependent on this PR; I think it should be fine, but it might change depending on discussions in this one, so could we maybe close that one and land that change as part of this?

Re “don’t land on v14.x,” is that because the version of V8 in 14.x doesn’t support unflagged assert?

[aduh95]

Re “don’t land on v14.x,” is that because the version of V8 in 14.x doesn’t support unflagged assert?

Correct, it doesn't support assert syntax at all (that was added in V8 9.1 IIRC, Node.js v14.x uses V8 8.4).

[GeoffreyBooth]

@aduh95 and I discussed this. Here’s what I think are the to-do items to land this PR:

• See if we can keep the import assertions out of the module cache key. Currently the resolve hook’s only required return value is the resolved module URL, which becomes the key; and then load returns the source, which is the module cache value. This gets significantly more complicated if we have a combined key of URL plus assertion, so I’d like to see if we can avoid that if at all possible.

• Validating the assertion should be isolated in its own function, so that we can determine where it should be called from.

• The validation logic should probably be called from within defaultLoad, so that it can be overridden/ignored by user loaders (we should discuss this with the larger loaders group and especially @JakobJingleheimer and @bmeck at next week’s loaders meeting).

• The assertions need to be passed into the resolve hook on its context object, and probably also into the load hook on its context object. We could also allow user resolve hooks to optionally return an assertions object, which would then replace (?) the load hook’s context.assertions. This is something else we should discuss in the next loaders meeting.