new Array（）vs Object.create（Array.prototype） [英] new Array() vs Object.create(Array.prototype)
问题描述
天真的困惑：
var arr1 = new Array();
var arr2 = Object.create(Array.prototype);
//Inserting elements in "both arrays"
arr1[0] =0;
arr1[9] =9;
arr2[0] =0;
arr2[9] =9;
arr1.push(10);
arr2.push(10);
console.log(arr1.length); // prints 11
console.log(arr2.length); // prints 1
两个对象都继承了Array.prototype，但它们与[]运算符的行为不同。为什么？
Both objects inherits Array.prototype, but they behave differently with the [] operator. Why?
推荐答案
在第一种情况下，您创建一个维护长度的数组对象属性。
In the first case you create an array object that maintains the length property when you access a integer, non-negative property (index).
在第二种情况下，您创建了一个常规对象继承 Array 原型。在该对象上使用 [] 与任何对象相同，只需在其上设置常规属性。
In the second case you created a regular object that inherits the Array prototype. Using [] on that object is the same as any object and simply sets regular properties on it.
var arr1 = new Array(); // or var arr1 = [];
arr1[0] = 0;
arr1['foo'] = 3;
// arr1 has a length of 1 because 0 is an array index and 'foo' is a regular property.
var arr2 = Object.create(Array.prototype);
arr2[0] = 0;
arr2['foo'] = 3;
// arr2 has a length of 0 because both 0 and 'foo' are regular properties.
ECMAScript 5语言规范描述长度的方式保存在第15.4节中。
The ECMAScript 5 Language Spec describes how length is maintained in section 15.4.
数组对象对某类属性
名称给予特殊处理。属性名称 P （以String值的形式）是数组索引当且仅当ToString（ToUint32（ P ））是等于 P 和
ToUint32（ P ）不等于2 ^（32-1）。
Array objects give special treatment to a certain class of property names. A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2^(32−1).
[...]
具体来说，每当添加
属性时，其名称为数据索引，长度属性是
，如果需要，改为比
数组索引的数值多一个;
Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index;
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