Undocumented inability to put REPL in ES module context

[DerekNonGeneric]

There does not appear to be a way to put the REPL in ES module context.

The closest you'll get to it seems to be by running:

1. node --experimental-repl-await
2. await import('./foo.mjs')

Give it a try and you'll see that evaluating import.meta throws this error:

import.meta
       ^^^^

Uncaught SyntaxError: Cannot use 'import.meta' outside a module

I'd like this issue to discuss whether or not this limitation is presently documented somewhere or should be illuminated in https://nodejs.org/api/esm.html or elsewhere.

Note: this issue isn't about executing code with --eval using the --input-type=module flag.

[GeoffreyBooth]

cc @devsnek, what's the latest on that "REPL" parse goal?

[devsnek]

if you mean the tc39 proposal, it's not doing anything right now. if you mean the v8 debug feature, I think they're in user feedback mode atm, not actively making anything.

[GeoffreyBooth]

So I think where we left off with this was that when/if the REPL parse goal shipped, our REPL would use that. But if that's not advancing, should we discuss alternatives?

One option that comes to mind is to just extend --input-type to apply to the REPL. But I don't know how that would work in practice; I would think that static import and export statements would throw, for example. And since import() is already supported in Script/CommonJS, such a REPL mode wouldn't be that different than what we have now.

I tried typing import.meta into the Chrome console and I got the same Uncaught SyntaxError: Cannot use 'import.meta' outside a module error, so I guess that's not supported there either.

[devsnek]

In the short term I can try to look into enabling import declarations and import.meta in V8 replMode. I don't think --input-type should have any effect on the repl, it doesn't really make sense to call it a script or a module or anything like that. We will have to overhaul our repl in node to take advantage of replMode though (cc @BridgeAR)

[GeoffreyBooth]

So V8 replMode is basically the “REPL parse goal” I remember you describing, where it tries to support everything at once? import statements, redeclaring const, etc.?

[devsnek]

@GeoffreyBooth i don't know the exact plans of the v8 team, i posted in v8:6903 so we'll see what happens.

[BridgeAR]

The replMode allows to reassign let statements gracefully. We'll have to implement some parts in vm first to access that part though (at least that's how it seemed when I last checked).

It is likely that the replMode will receive further changes later on but that's all that is included so far.

[BridgeAR]

Top level await will be available as soon as V8 supports that (acorn must also support it but that should not take too long afterwards). It's independent from the replMode.

[DerekNonGeneric]

Should we document this inability in the meantime? Hope to tackle this issue unless it's tracking something else.

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