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）1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*k2 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 さんのツイートを参考にしました。