JavaScript对象创建/关联性

Question

有人能解释一下以下代码片段的输出行为吗？

function Vertex(x, y, z) {
    this.x = parseInt(x);
    this.y = parseInt(y);
    this.z = parseInt(z);
};

var set = {};

var v1 = new Vertex(1,2,3);
var v2 = new Vertex(2,3,4);
var v3 = new Vertex(1,2,3);
var v4 = new Vertex(4,5,6);

set[v1] = 10;
set[v2] = 11;


alert(set[v1] + " " + set[v2] + " " + set[v3] + " " + set[v4]);
// Output: 11 11 11 11

Answer 1

v1 == v2 == v3 == v4 == "[object Object]" when expressed as a string

您不能将对象作为键。

set[v1] is the same as set["[object Object]"]

这就是为什么你会得到所有4个键的最后一个值。

这里的是您重新编写的代码，以便按预期执行

var object_id = 0; /* global object instance id */
function Vertex(x, y, z) {
    this.x = parseInt(x);
    this.y = parseInt(y);
    this.z = parseInt(z);
    this.id = (object_id++); /* add id to object */
};
/* override to string so it can be used as a key */
Vertex.prototype.toString = function() {
    return JSON.stringify(this);
}

var set = {};

var v1 = new Vertex(1,2,3);
var v2 = new Vertex(2,3,4);
var v3 = new Vertex(1,2,3);
var v4 = new Vertex(4,5,6);

set[v1] = 10;
set[v2] = 11;


alert(set[v1] + " " + set[v2] + " " + set[v3] + " " + set[v4]);

Answer 2

对象属性是字符串，因此如果您尝试执行set[v1] = 10;，v1将被转换为字符串(字符串将为[object Object])，因此本质上您将获得set['[object Object]'] = 10;这将发生在您的所有对象上，并且您设置的每个值都将覆盖先前的值。

所以你的代码等同于这个

function Vertex(x, y, z) {
    this.x = parseInt(x);
    this.y = parseInt(y);
    this.z = parseInt(z);
};

var set = {};

var v1 = new Vertex(1,2,3);
var v2 = new Vertex(2,3,4);
var v3 = new Vertex(1,2,3);
var v4 = new Vertex(4,5,6);

set['[object Object]'] = 10;
set['[object Object]'] = 11;          

alert(set['[object Object]'] + " " + set['[object Object]'] + " " + set['[object Object]'] + " " + set['[object Object]']);
// Output: 11 11 11 11