[[][+[]]+[]][+[]][++[[]][+[]]]

この画面で、以下を順に行ってください

- option + command + I
- `[[][+[]]+[]][+[]][++[[]][+[]]]` をコピー
- 右の画面にペースト
- 出力結果を確認

```
> [[][+[]]+[]][+[]][++[[]][+[]]]
< 'n'
```


順に説明します

## `+[]` → `0`
JavaScript では、`Array` に `+` 演算子は定義されていません。よって、`+[]` は `+String([])` と解釈されます。

`String([1, 2, 3]) = '1,2,3'` なので、`String([]) = ""` となります。


また、`""`（空文字列）は、**Falsey（偽値）**[^falsey]です。

> 偽値 (falsy または falsey な値) は、 Boolean コンテキストに現れたときに偽とみなされる値です。

また、単項演算子としての `+` は、`boolean` 型を `number` 型にキャストします。
よって、`+[]` → `+""` → `+false` → `+0` → `0` となります。

タイトルの文字列の `+[]` を `0` に置き換えると、以下のように変換されました。

`[[][0]+[]][0][++[[]][0]]`

## `[][0]+[]` → `"undefined"`
`[][0]` は、空配列の 0 番目の要素という意味であり、要素が存在しないので `undefined` になります。

JavaScript では、`Array` や `undefined` に `+` 演算子は定義されていません。よって、`[]` は `String([])` に、`undefined` は `String(undefined)` と解釈されます。
よって、`undefined + []` は、`String(undefined) + String([])` → `"undefined" + ""` → `"undefined"`（文字列）と解釈されます。

タイトルの文字列は以下のように変換されました。
`["undefined"][0][++[[]][0]]`

## `++[[]][0]` → `1`
`[[]][0]` は、`[[]]` の 0 番目の要素を表すので、`[]` を意味します。`[]` を `A` とすると、`++A` は、「`A` に 1 を加算した値を返す」ので、`++A` は `+A + 1` に等しいです。

`+[]` は `0` と等しいので、`[[]][0]` → `1` となります。

タイトルの文字列は以下のように変換されました。
`["undefined"][0][1]`


## `["undefined"][0][1]` → `'n'`

お分かりですね？


## おまけ
この記事では、`[`, `]`, `+` の 3 種類の文字のみを用いて色々やっていますが、`(`, `)`, `!` を追加すると表現力がかなり高まるそうです。

JSFu*k[^fck] というサイトで、`"monkukui"` という文字列を表す JS コードを生成してみた結果、以下のようになりました。

```
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```

わお

本記事は、[@tempura_cpp](https://twitter.com/tempura_cpp) さんの[ツイート](https://twitter.com/tempura_cpp/status/1468875441583046658?s=20)を参考にしました。


[^falsey]: https://developer.mozilla.org/ja/docs/Glossary/Falsy

[^fck]: http://www.jsfuck.com