ECMAScript仕様輪読会 #96

Twitter hashtag: #esspec

便利ツール

esspec-memo: https://tars0x9752.github.io/esspec-memo/esspec.html

Scrapbox Codeblock Generator: https://vzvu3k6k.github.io/scrapbox-codeblock/

TC39 Terminology: https://github.com/tc39/how-we-work/blob/main/terminology.md

esmeta playground: https://es-meta.github.io/playground/

時事ネタ

自己紹介 (近況報告)

syumai syumai.icon

X: https://x.com/__syumai GitHub: https://github.com/syumai

TSを書いて暮らしてます

TSKaigi Proposal出しました

iwatsurut

とくに、イベントもなく過ごしています。

今日 Hacker News に載ってました。夏時間だけにするらしい。

https://www.cbc.ca/news/canada/british-columbia/b-c-adopting-year-round-daylight-time-9.7111657

igrep(山本悠滋)

https://github.com/igrep/

React Tokyo Fes 2026のスポンサーブースめっちゃ忙しかった

maru。（まる）

https://x.com/sbleru

https://github.com/sbleru

主にTSで仕事してます。React, Node。

mcp周りの実装してた

memo

https://ai-sdk.dev/docs/introduction

https://zenn.dev/takuh/articles/6de1a0f6046dd9

前回のあらすじ

https://syumai.github.io/esspec/summaries/summary-95.html

今回のメモ

ふぁぼんさんがすごい

https://x.com/syobon_hinata

https://github.com/fabon-f/temporal-polyfill-lite

code:js

new Date(2026, 0, 1, 0, 0, 0, 0)

2025-12-31T15:00:00.000Z

Date.UTC(2026, 0, 1, 0, 0, 0, 0)

1767225600000

new Date(Date.UTC(2026, 0, 1, 0, 0, 0, 0))

2026-01-01T00:00:00.000Z

Date.UTC(273790, 0, 1, 0, 0, 0, 0)

8577810748800000

Date.UTC(285400, 0, 1, 0, 0, 0, 0)

NaN

Date.UTC(280000, 0, 1, 0, 0, 0, 0)

NaN

これ？

code:js

// 最大が 8.64 × 10**15

Date.UTC(275760, 8, 13, 0, 0, 0, 0)

8640000000000000

Date.UTC(275760, 8, 13, 0, 0, 0, 1)

NaN

code:js

(new Date()).getDate()

const date = new Date()

undefined

getDate.call(date)

getDate.call("string")

Uncaught TypeError: this is not a Date object.

at String.getDate (<anonymous>)

const nanDate = new Date(NaN)

undefined

nanDate

Invalid Date

nanDate.getDate()

NaN

code:js

date.getTime()

1772536306887

nanDate.getTime()

NaN

nanDate.getTimezoneOffset()

NaN

date.getTimezoneOffset()

-540

date.getTimezoneOffset()/60

-9

code:js

d3.setDate(28)

1803776461001

2027-02-28T01:01:01.001Z

d3.setDate(29)

1803862861001

2027-03-01T01:01:01.001Z

d3.setDate(-1)

1803690061001

2027-02-27T01:01:01.001Z

d3.setDate(0)

1801357261001

2027-01-31T01:01:01.001Z

code:js

date.setTime(Number.MAX_SAFE_INTEGER)

NaN

date

Invalid Date

date.getTime()

NaN

code:js

const d2 = new Date(2026, 0, 1, 9, 0, 0, 0);

undefined

2026-01-01T00:00:00.000Z

d2.toLocaleString()

'1/1/2026, 9:00:00 AM'

d2.setDate(2)

1767312000000

2026-01-02T00:00:00.000Z

d2.toLocaleString()

'1/2/2026, 9:00:00 AM'

d2.setUTCDate(3)

1767398400000

2026-01-03T00:00:00.000Z

d2.toLocaleString()

'1/3/2026, 9:00:00 AM'

const d3 = new Date(2026, 0, 2, 0, 0, 0, 0);

undefined

2026-01-01T15:00:00.000Z

d3.toLocaleString()

'1/2/2026, 12:00:00 AM'

d3.setDate(3)

1767366000000

2026-01-02T15:00:00.000Z

d3.toLocaleString()

'1/3/2026, 12:00:00 AM'

d3.setUTCDate(4)

1767538800000

2026-01-04T15:00:00.000Z

d3.toLocaleString()

'1/5/2026, 12:00:00 AM'

ブラウザでの挙動

code:js

const d3 = new Date(2026, 0, 2, 0, 0, 0, 0);

undefined

d3.toLocaleString()

'2026/1/2 0:00:00'

const d3 = new Date(2026, 0, 2, 9, 0, 0, 0);

undefined

d3.toLocaleString()

'2026/1/2 9:00:00'

const d3 = new Date(2026, 0, 2, 22, 0, 0, 0);

undefined

d3.toLocaleString()

'2026/1/2 22:00:00'

code:js

❯ node -v

v24.14.0

const d3 = new Date(2026, 0, 2, 0, 0, 0, 0);

undefined

d3.toLocaleString()

'2026/1/2 0:00:00'

code:js

2026-01-04T15:00:00.000Z

d3.toDateString()

'Mon Jan 05 2026'

d3.setTime(NaN)

NaN

Invalid Date

d3.toDateString()

'Invalid Date'

code:js

const d4 = new Date(275760, 0, 1, 0, 0, 0, 0)

undefined

+275759-12-31T15:00:00.000Z

d4.toISOString()

'+275759-12-31T15:00:00.000Z'

code:js

const obj = {

| toISOString() { return "dummy"; }}

undefined

Date.prototype.toJSON.call(new Date())

'2026-03-03T12:30:53.035Z'

Date.prototype.toJSON.call(obj)

'dummy'

const obj2 = { Symbol.toPrimitive() { return Infinity },

| toISOString() { return "dummy2"; }}

undefined

Date.prototype.toJSON.call(obj2)

null