javascript

Copying Values
Chapter 4: Variables, Scope, and Memory
Aside from differences in how they are stored, primitive and reference values act differently when
copied from one variable to another. When a primitive value is assigned from one variable to another,
the value stored on the stack is created and copied into the location for the new variable. Consider the
following example:
var num1 = 5;
var num2 = num1;
Here, num1 contains the value of 5. When num2 is initialized to num1 , it also gets the value of 5. This value
is completely separate from the one that is stored in num1 because it ’ s a copy of that value. Each of these
variables can now be used separately with no side effects. This process is diagrammed in Figure 4-2 .
Stack before copy
num1
5
(Number type)
Stack after copy
num2
num1
5
(Number type)
5
(Number type)
Figure 4 - 2
When a reference value is assigned from one variable to another, the value stored on the stack is also
copied into the location for the new variable. The difference is that this value is actually a pointer to an
object stored on the heap. Once the operation is complete, two variables point to exactly the same object,
so changes to one are reflected on the other, as in the following example:
var obj1 = new Object();
var obj2 = obj1;
obj1.name = “Nicholas”;
alert(obj2.name); //”Nicholas”
In this example, the variable obj1 is filled with a new instance of an object. This value is then copied into
obj2 , meaning that both variables are now pointing to the same object. When the property name is set
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