javascript - new Array() 与 Object.create(Array.prototype)

Question

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

这两个对象都继承了 Array.prototype，但它们使用 [] 运算符的行为不同。为什么？

Answer 1

在第一种情况下，当您访问整数、非负属性(索引)时，您创建了一个数组对象，该对象保持 length 属性。

在第二种情况下，您创建了一个继承 Array 原型(prototype)的常规对象。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即可。

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 语言规范描述了如何在 section 15.4 中维护 length .

Array objects give special treatment to a certain class of propertynames. 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 andToUint32(P) is not equal to 2^(32−1).

[...]

Specifically, whenever aproperty is added whose name is an array index, the length property ischanged, if necessary, to be one more than the numeric value of thatarray index;