javascript - 为什么要通过括号访问 Symbol.iterator？

Question

如果我创建了一个数组，例如，var array=[1,2,3,4]; 来返回一个迭代器，我会做 var iterator = array[Symbol.iterator ](); 不明白为什么要通过括号访问Symbol.iterator属性？为什么不只是 array.Symbol.iterator？

Answer 1

名为 Symbol 的数组没有属性(除非你把一个放在那里)。相反，您正在查找其键是 Symbol.iterator 指向的符号原语的值。 Symbol.iterator 返回一个符号，您使用该符号作为查找键。这有点像查找带有变量的属性:

let a = [1, 2, 3]
a.someProp = "hello"

let key = "someProp"

// this doesn't work for the same reason s.Symbol.iterator doesn't:
// a.key

// but this does:
console.log(a[key])

// So with a Symbol:

let k = Symbol.iterator
console.log(typeof k)
// k is now a reference to the symbol that is the key
// you can use that to returns the iterator function
console.log(a[k])    // <-- that returns your iterator function
console.log([...a[k]()])

// equivalent to:
console.log([...a[Symbol.iterator]()])