.mjs
2018/04/24 We are JavaScripters!!
@ Recruit Technologies

Twitter: @yosuke_furukawa
Github: yosuke-furukawa

Node.js v10 Release🎉
(maybe tomorrow)

Node.js v10
Notable Changes
• for-await-of in Stream experimental support
• ESModules experimental support
• private/public class fields experimental
support
• fs/promises

DEMO

DEMO
import fs from 'fs/promises';
import util from 'util';
async function main() {
try {
const content = await fs.readFile(process.argv[2])
console.log(content.toString())
} catch(e) {
console.error(e.toString());
}
}
main();

file name is … foo.mjs

＿＿人人人＿＿
＞ .mjs !!! ＜
￣￣Y^Y^Y￣￣

Yes, we have long
discussion about ESM.

We need ".mjs".

Why?

Module !== Script

Module
Script
[
[
"strict",
"non strict",
"top level scope", "global scope",
"reserved await"
]
]

We need to detect which
mode on load file.

if file.lastIndexOf(".mjs") {
Module
} else {
Script
}

Simple and Faster

We do not use ".js" ????
(´・ω・｀)

No, 2 reasons.

Use loader option.

DEMO

DEMO
const builtins = Module.builtinModules;
const URL = url.URL;
const baseURL = new URL('file://');
baseURL.pathname = `${process.cwd()}/`;
export function resolve(specifier, parentModuleURL = baseURL, defaultResolve) {
if (builtins.includes(specifier)) {
return {
url: specifier,
format: 'builtin'
};
}
const resolved = new url.URL(specifier, parentModuleURL);
const ext = path.extname(resolved.pathname);
return {
url: resolved.href,
format: 'esm'
};
}

DEMO
$ node —experimental-modules —loader loader.mjs file.js
// file.js
import fs from 'fs';
import util from 'util';
const readFile = util.promisify(fs.readFile);
async function main() {
try {
const content = await readFile(process.argv[2])
console.log(content.toString())
} catch(e) {
console.error(e.toString());
}
}
main();

mode detection on flag
`—mode` , package.json
see:
https://gist.github.com/ceejbot/b49f8789b2ab6b09548ccb72813a1054

https://docs.google.com/presentation/d/
1xK1ana_TIxfAHX33CYVHFnJsV0If_YirLtRBBga9cv0/edit#slide=id.p

You just use ".js" if you
don’t need to use ESM.

You would be better to use
".mjs", if you need ESM on
Node.js v9-10.

You would be better to wait
the conclusion, if you want
to use ES Modules in the
future.

蛇足 (da-soku)
🐍 + 👣

Filename Extensions
YOMO-YAMA

Long long time ago…
Brendan Eich saids …

How do we think .es3, .es4 file extensions to detect
versions?
We need to change filename extensions to upgrade
??? that is nightmare ….
// sorry no reference… I cannot find link

Recently, I met with the Google V8 team for two full
days. One message that came through loud and
clear, that I said I would relay to the list, is "please,
no more modes."
I dislike modes. Who doesn't? But this sounds like
the "Versioning is an anti-pattern" shibboleth I've
heard recently re: WebSockets. It does not reflect
reality.
https://esdiscuss.org/topic/no-more-modes

Web does not have
version/mode.

All we need is feature
detection.

Detectable Feature
// Feature Detection
if (!Array.isArray) {}
if (!String.prototype.trimStart) {}

un-detectable feature (on runtime)
class A {}
import / export
#foo
// syntax

Modern JavaScript is
difficult to detect features.

Module JavaScript is
JavaScript 2.0

JavaScript is not so easy,
We are JavaScripters,
Every JSers would be better
to enjoy chaos.

Enjoy JS Chaos :)

Thanks