关于ES6数组破坏和交换的有趣行为

Question

我刚刚注意到，以下代码(在let [one, two] = [1, 2]之后没有立即引用let [one, two] = [1, 2])尝试[one, two] = [two, one]会崩溃：

​

let [one, two, three] = [1, 2, 3]
[one, two] = [two, one] // CRASH! one is not defined
console.log(one, two)

​

但是，简单地在声明和交换之间添加一个未使用的one突然允许代码，但不正确：

​

let [one, two, three] = [1, 2, 3]
one // inserted here
[one, two] = [two, one] // no longer crashes! but not actually swapping
console.log(one, two) // should be '2 1', but shows '1 2' instead

​

而下面的代码给出了预期的交换效果

var a = 1;
var b = 3;

[a, b] = [b, a];
console.log(a); // 3
console.log(b); // 1

​

有人能解释为什么这样的行为存在吗？谢谢!

Answer 1

因为它被解析成这样：

 let [one, two, three] = [1, 2, 3][one, two] = [one, two]

要使它发挥作用，总是用父母来破坏任务：

 let [one, two, three] = [1, 2, 3];
 ([one, two] = [two, one]);
 console.log(one, two);

永远不要相信ASI，如果它出了问题，就会出现这样的情况。