javascript - Node.js v8 HOLEY 数组意外行为

Question

看完Mathias Bynens关于 HOLEY 和 PACKED 阵列的报告，我做了一些实验并得到了意想不到的行为。看两种情况:

// CASE #1
const array = [0,1,2,3,4,5,undefined]
array.__proto__[6] = 'some value'

console.log(array[6]) // "some value" expected but got undefined

// CASE #2
const array = [0,1,2,3,4,5]
array[10] = 'faraway value'
array.__proto__[6] = 'some value'

console.log(array[6]) // "some value" expected and received

那么，这些情况之间有什么区别？为什么在第一种情况下它不查看原型(prototype)链就返回 undefined ？

Answer 1

在案例 #1 中，您明确地将 undefined 放入数组中。没有必要沿着原型(prototype)链往上走，因为元素已经存在。比较:

var case1 = [0, 1, 2, 3, 4, 5, undefined, 7];
var case2 = [0, 1, 2, 3, 4, 5, , 7];
case1.hasOwnProperty(6);  // true
case2.hasOwnProperty(6);  // false
console.log(case1[6]);  // "undefined", loaded from object
console.log(case2[6]);  // "undefined", because no element was found (not on the object, and not on the prototype chain)
case2.__proto__[6] = "proto value";
console.log(case2[6]);  // "proto_value", loaded from prototype chain