problem with default export

[Gozala]

@vmx had reported a problem with default export not passing a type check

multiformats/js-multiformats#66 (comment)

import CID from 'multiformats/cid
import raw from 'multiformats/codecs/raw

In commonjs modules, these kind of imports don't work (same with and without this PR). When I do

const CID = require('multiformats/cid')

I get

error TS2339: Property 'parse' does not exist on type 'typeof import("/some/path/js-multiformats/dist/types/cid")'

I also tried adding

/**
 * @typedef {import('multiformats/cid')} CID
 */

but that didn't help either.

The problem here is that TS expects CID = require('multiformats/cid').default instead. Here is how they put it:

By default (with esModuleInterop false or not set) TypeScript treats CommonJS/AMD/UMD modules similar to ES6 modules. In doing this, there are two parts in particular which turned out to be flawed assumptions:

• a namespace import like import * as moment from "moment" acts the same as const moment = require("moment")
• a default import like import moment from "moment" acts the same as const moment = require("moment").default

This mis-match causes these two issues:

• the ES6 modules spec states that a namespace import (import * as x) can only be an object, by having TypeScript treating it the same as = require("x") then TypeScript allowed for the import to be treated as a function and be callable. This breaks the spec’s recommendations.
• while accurate to the ES6 modules spec, most libraries with CommonJS/AMD/UMD modules didn’t conform as strictly as TypeScript’s implementation.

In theory esModuleInterop option should fix this, but that does not seem to work in practice.

I think we need to either:

1. Change how ipjs generates .cjs files, which is either:
2. Default export goes into exports.default =
3. Or we do module.exports = thing; thing.default = thing; (I don't like this option)
4. Just use and support named exports (I'm sure anything else would break some assumption some other tool makes)
5. Make type generation a job of ipjs instead. And generate two sets of types (this will probably also address Publishing from dist breaks "types" and "typeVersions" fields of package.json #10)
   1. From and for ES modules
   2. From and for cjs modules