变量/对象是否按值传递，为什么我不能在javascript中使用变量来更改对象的属性？ [英] Are variables/objects passed by value and why can't I change object's property with variable in javascript?

问题描述

假设我有一个对象：

  var obj = {
 len：4，
 bred：5 
} 
  

现在假设我将这个对象赋值给一个变量 x 作为 var x = obj; 。据我了解，它创建了一个 obj 的副本，并将该副本分配给 x - 这是通过值。现在，如果我更改 x 的属性，那么它也会更改obj对象的属性。例如

  x.len = 99 
  

然后 obj.len 和 x.len 变成 99 。另一方面考虑这种情况：

  var r = 2，s = 3，t = 4; 
 s = r; 
 s = 88; 
  

现在 r 按值传递给 s s 一份 r 赋予<$ ç$ C>取值。因此，将 s 更改为 88 不会更改 r 变量。在控制台输入 r 仍然会给出 2 。

问题1 ：如果变量（包含的对象）在JavaScript中按值传递，那么为什么更改 x.len 更改原始 obj.len ，也是吗？

另一个问题是，我无法在分配给变量时更改对象的属性。考虑这种情况：

  var obj2 = {
 len：4，
 bred：5 
} 
 var x2; 
 x2 = obj.len; 
  

现在在控制台中输入 x2 code> 4 。但是，如果我尝试更改 x2 的值，例如 x2 = 77; ，那么不会更改 obj2.len 。

问题2：为什么我无法使用变量更改对象的属性？

解决方案

所有东西都是按值传递的，但是当您创建一个对象时，您将获得对该对象的引用。

  //创建一个对象。将该对象的引用赋值给`obj` 
 var obj = {
 len：4，
 bred：5 
}; 
 //将`obj`的值复制到`x`。现在`x`也是该对象的引用。 
 var x = obj; 
  

对 x 或 obj 现在将修改同一个对象，因为它们通过同一个引用的副本。

---

  var obj2 = {
 len：4，
 bred：5 
} 
 var x2; 
 x2 = obj.len; 
  

len 是一个数字，不是对象，所以这个值不是一个参考。所以，当你复制它时，你会得到一个数字的副本（而不是该对象的引用的副本）。

Suppose I have an object as:

var obj = {
        len: 4,
        bred: 5
    }

Now suppose I assign this object to a variable x as var x = obj;. As far as I understand it, it creates a copy of obj and assign that copy to x — that is pass by value. Now If I change a property of x then it changes that property of the obj object too. E.g.

x.len = 99

Then both obj.len and x.len become 99. On the other hand consider this scenario:

var r = 2, s = 3, t = 4; 
s = r;
s = 88;

Now r is passed by value to s that s a copy of r was given to s. So changing s to 88 doesn't change the original value of r variable. Typing r in the console still gives 2.

Question 1: If variables (objects included) are passed by value in JavaScript then why does changing x.len change the original obj.len, too?

Another problem is that I cannot change an object's property when assigning to a variable. Consider this scenario:

var obj2 = {
        len: 4,
        bred: 5
    }
var x2;
x2 = obj.len;

Now typing x2 in console simply returns 4. But if I try to change the value of x2 such as x2 = 77; then that doesn't change obj2.len.

Question2: Why can't I change object's property with a variable?

解决方案

Everything is passed by value, but when you create an object you get an reference to that object.

// Create an object. Assign a reference to that object to `obj`
var obj = {
    len: 4,
    bred: 5
};
// Copy the value of `obj` to `x`. Now `x` is also a reference to that object.
var x = obj;

Any changes to properties of x or obj will now modify the same object because they go through copies of the same reference.

---

var obj2 = {
    len: 4,
    bred: 5
}
var x2;
x2 = obj.len;

len is a number, not an object, so the value isn't a reference. So when you copy it you get a copy of the number (instead of a copy of the reference to the object).

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