javascript - 对空字符串的数组引用如何成为 JavaScript 中的有效字符？

Question

我不确定这行 javascript 中发生了什么:

alert( (''+[][[]])[!+[]+!+[]] ); // shows "d"

我发现了什么:

var a = ! + []; // == true
var b = ! + [] + ! + []; // == 2

似乎第二部分是对字母数组或某种形式的引用，但我不明白这是怎么来的

(''+[][[]])

还有:

alert( (''+[][])[2] ); // nothing happens; console says "unexpected token ]"
alert( (''+[[]][])[2] ); // nothing happens; console says "unexpected token ]"
alert( (''+[[]][[]])[2] ); // shows "d"
alert( (""+true)[2] ); // shows "u"

Answer 1

我帮你分解一下:

  ('' + [][[]])[!+[]+!+[]]
= ('' + undefined)[!+[]+!+[]]  // [][[]] grabs the []th index of an empty array.
= 'undefined'[! + [] + ! + []]
= 'undefined'[(! + []) + (! + [])]
= 'undefined'[true + true]
= 'undefined'[2]
= 'd'

! + [] == true 在这里解释 What's the significant use of unary plus and minus operators?