* 仕様書

[REFS[
- [2] [CITE[HTML Standard]]
]REFS]

* 取得器

[3] [CODE(DOMi)@en[History]] [[インターフェイス]]の 
[DFN[[CODE(DOMa)@en[index][history.index]]]] [[属性]]の[[取得器]]は、
次のようにしなければ[MUST[なりません]] [SRC[>>2]]。

[FIG(steps)[
= [5] [[文脈オブジェクト]]の[F[文書]]が[F[完全に活性]]で''ない''場合、
== [6] [CODE[SecurityError]] [CODE[DOMException]] を[[投げ]]、ここで停止します。
= [4] [[結合セッション履歴の現在エントリー]]の、
[[最上位閲覧文脈]]の[F[結合セッション履歴]]における[[索引]]を返します。
]FIG]

* 歴史

[1] [CITE@en[Add history.index]]
([[domenic]]著, [TIME[2017-08-19 06:27:37 +09:00]])
<https://github.com/whatwg/html/commit/f8e293fa081b29676405507b998ff2cbd9f41e42>

[7] [CITE@en[Expose `history.index` in addition to `history.length` · Issue #2710 · whatwg/html]]
([TIME[2017-08-24 11:49:12 +09:00]])
<https://github.com/whatwg/html/issues/2710>

[8] [CITE@en[Add history.index by domenic · Pull Request #2944 · whatwg/html]]
([TIME[2017-08-24 11:49:28 +09:00]])
<https://github.com/whatwg/html/pull/2944>

[9] [CITE@en[Fix typo in previous commit]]
([[domenic]]著, [TIME[2017-08-23 13:49:06 +09:00]])
<https://github.com/whatwg/html/commit/f8a212406c906bd72788cbea6ebd60f4bdb087a5>

[10] [CITE@en[Fix typo in previous commit by domenic · Pull Request #2960 · whatwg/html]]
([TIME[2017-08-24 11:50:02 +09:00]])
<https://github.com/whatwg/html/pull/2960>

[11] [CITE@en[Remove history.index]]
([[domenic]]著, [TIME[2018-02-08 04:52:07 +09:00]])
<https://github.com/whatwg/html/commit/430a6531dd6cb25c9fe7903c823e86eabfde784c>

[12] [CITE@en[Remove history.index by domenic · Pull Request #3460 · whatwg/html]]
([TIME[2018-02-17 18:20:55 +09:00]])
<https://github.com/whatwg/html/pull/3460>

[13] [CITE@en[Session history indexing is broken · Issue #2985 · whatwg/html]]
([TIME[2018-02-17 18:22:12 +09:00]])
<https://github.com/whatwg/html/issues/2985>