Should `undefined` allowed in second assignment of import call?

[tc39]

Should `undefined` allowed in second assignment of import call?

[littledan]

This proposal is going to make trailing commas allowed in import().

[devsnek]

I guess I don't see any real reason to police the number of arguments passed.

[jridgewell]

I don't understand why we treat import special. Just give it arguments. Who cares if you pass 0 args, 1 arg, or 100. If the first arg can be resolved to a specifier, then you're fine. And if the second arg is provided, and its a real object, you're fine.

That import is a special keyword shouldn't change they way we process the arguments you pass to it. And it certainly shouldn't make import(foo, bar) behave differently if bar resolved to undefined.

[ljharb]

That seems like it'd allow a bug; why not the following?

const assets = [
  ["ad-banner.mjs"],
  ["i18n/zh.json", { type: "json" }]
];

for (const [path, attrs] of assets) {
  await attrs ? import(path, attrs) : import(path);
}

[devsnek]

by convention we treat undefined and no argument the same now, it should be allowed.

[JLHwung]

@ljharb The attrs can be an empty string but I am not gonna nit-picking.

Since the second argument is optional, developers may align it mentally to function calls where passing undefined is almost same as no arguments, except for arguments or other edge cases.

[littledan]

I agree with @devsnek , that undefined should be treated the same as no second parameter. On the other hand, I'd expect null to cause a TypeError due to getting the with property from it.

In the current draft in the explainer, you'd use { with: { type: "json" } } as the second argument to import(). We could discuss changing this in a separate issue, if you have concerns about it.

[ljharb]

@devsnek not for import(), or import(a,) would be allowed.

If, however, this proposal is going to make trailing commas in import() allowed, then i agree it should match with function call expectations.

[devsnek]

Why not just go all in and use |Arguments|

[JLHwung]

@devsnek I think the concern here is still valid for import(a, b): Supporting SpreadElement requires runtime error check on argument length.

[ljharb]

It's not a function tho, and it's very important it not be treated as one. Functions can be passed around, import() can't, and functions - most importantly - do not behave differently based on where they are invoked, while import() can.

[jridgewell]

We invoke it like a function, and it returns a value like a function. Whether the callee is a real value or a syntax form shouldn't affect the way we treat its arguments.

[littledan]

I see I thumbs-upped the comment on the restriction initially, but I'd be OK with weakening it to general |Arguments|, or keeping the two-arg, no-spread restrictions as in the current spec outline. I see this as a post-Stage 2, non-core bikeshed, since there are multiple options that would each be non-fatal, and the task is to figure out which is best.