Support node v13 with es modules

[vikerman]

Creates conditional export that points "import" conditional exports to mjs files. This lets node load the esm version without having to set "type": "module" on the main package.json.

This is similar to the approach in the main preact repo - preactjs/preact#2451

[hybrist]

One note of caution is that this is a breaking change: People who currently require or import ./dist/ paths directly would no longer be able to. This can be softened by adding an export of “./“.

Excited to see this added to htm!

[kristoferbaxter]

Is this a good opportunity to consider dropping umd for the export map? Or perhaps in a separate PR.

I'm not sure I see the value in specifying a umd format output in export maps.

[vikerman]

I was just following the pattern from the main preact repo. I can do the change if needed.

[kristoferbaxter]

No worries, I think this is a good followup item to consider. Not blocking for this PR.

Thanks for doing the work!

[vikerman]

Looks like the node version changed and the test is failing because npm cache has older version of compiled iltorb. Clearing the npm cache in Travis could help

[developit]

@vikerman - just switched to Actions, hopefully that addresses it.

[developit]

Everything looks good. I'm wondering if there's a way to test this though.
Something like:

{
  "scripts": {
    "test:dist": "npm pack && mv htm*.tgz test/fixtures/dist/htm.tgz && cd test/fixtures/dist && node ."
  }
}

... where dist/ would be:

// package.json
{ "dependencies": {"htm": "file:htm.tgz" } }

// index.mjs
import 'htm';
import 'htm/preact';
import 'htm/preact/standalone';
import 'htm/react';

[vikerman]

Added the test. It's not hooked up to CI though. Would work for node versions >=13. [Edit: It's hooked up to the CI now]

[vikerman]

Hooked up the ESM dist test to CI when node version is 14.x.

[developit]

Wow, thanks @vikerman that is absolutely perfect!