Question

我没有通过此代码获得预期值，请帮我识别此代码中的错误。我是java脚本的新手，我怀疑问题可能是由于对象创建或对象范围。我通过多种方式对其进行了调试，但无法确定问题。

另请告诉我，我创建的数据是否是JSON格式。如果不是等效的JSON格式是什么？

PS.id = -1;

PS.data = {
 formData:[]
};

PS.setDef = [];

PS.item = {};

if(cond){

PS.item.Name = "From";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[0].Name);

PS.item.Name = "To";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[1].Name);

}//here alerts are coming properly. getting 'from' and 'to' in alerts

PS.data.formData.push({"id":id++, "setDef":[PS.setDef[0]]});
PS.data.formData.push({"id":id++, "setDef":[PS.setDef[1]]});

//expected id 0 
alert(PS.data.formData[0].id);
//expected name 'from'
alert(PS.data.formData[1].setDef[0].Name);

//expected id 1
alert(PS.data.formData[1].id);
//expected name 'to'
alert(PS.data.formData[1].setDef[1].Name);

不是将0和1作为ID而名称为'from'和'to'，而是获得以下值

-1
0
to
to

更新1：此处PS表示已声明的页面范围，编辑警报以打印变量。

Answer 1

您的代码有几个逻辑错误和拼写错误。这是更正后的版本（已在**HERE**）

的评论中标记了更改

PS.id = -1;

PS.data = {
    formData:[]
};

PS.setDef = [];

if(cond){
    // *** HERE ***
    // Your code was creating just one object instance in
    // PS.item, pushing it, mutating and pushing it again
    // The "alert" call was apparently correct just because
    // it was done before mutating the object. Just moving
    // both alerts at the end would have shown the problem.

    PS.setDef.push({Name: "From", Type: "STRING"});
    alert(PS.setDef[0].Name);

    PS.setDef.push({Name: "To", Type: "STRING"});
    alert(PS.setDef[1].Name);

}

// **HERE**
// id++ increments a value but returns the value BEFORE the increment.
// Thus formData[0] is expected to have .id == -1, not 0.
// If you want the value AFTER the increment then use ++id.
PS.data.formData.push({"id":++id, "setDef":[PS.setDef[0]]});
PS.data.formData.push({"id":++id, "setDef":[PS.setDef[1]]});

//expected id 0 
alert(PS.data.formData[0].id);
//expected name 'from'
// *** HERE ***
// In the original code there is a typo (formData[1] instead of formData[0])
alert(PS.data.formData[0].setDef[0].Name);

//expected id 1
alert(PS.data.formData[1].id);
//expected name 'to'
alert(PS.data.formData[1].setDef[1].Name);

Answer 2

这里有两件事。首先是id，将代码更改为以下内容，

PS.data.formData.push({"id":++PS.id, "setDef":[PS.setDef[0]]});
PS.data.formData.push({"id":++PS.id, "setDef":[PS.setDef[1]]});

因为当你将id ++分配给id时你的id以-1开头，所以它首先分配当前id的值，然后增加id的值，因此得到-1和0。

其次，当您按下PS.item时，实际上是将引用传递给对象而不是实际对象。因此，当您更新PS.item的值时，它会反映在两个地方。尝试创建一个新对象并推送它，而不是覆盖旧对象。

在分配值之前，使用PS.item = {};创建新对象。这应该有用。

Answer 3

您刚刚在代码的这一部分中引用了两次PS.item：

PS.item.Name = "From";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[0].Name);

//changing the value of PS.item 
PS.item.Name = "To";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[1].Name);

您可以通过这种方式编写代码，并对结果感到满意：

//creating first instance
PS.item = {};

PS.item.Name = "From";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[0].Name);

//creating the second instance
PS.item = {};

PS.item.Name = "To";
PS.item.Type = "STRING";

PS.setDef.push(PS.item);
alert(PS.setDef[1].Name);

有了主意吗？

在Javascript中创建对象的异常行为

3 个答案: