javascript

Chapter 7: Anonymous Functions
contains a reference to the activation object for createComparisonFunction() . Figure 7 - 2 illustrates
this relationship when the following code is executed:
var compare = createComparisonFunction(“name”);
var result = compare({ name: “Nicholas” }, { name: “Michael” });
When the anonymous function is returned from createComparisonFunction() , its scope chain has
been initialized to contain its own activation object, the activation object from
createComparisonFunction() , and the global variable object. This gives the anonymous function
access to all of the variables from createComparisonFunction() . Another interesting side effect is that
the activation object from createComparisonFunction() cannot be destroyed once the function
finishes executing because a reference still exists in the anonymous function ’ s scope chain. After
createComparisonFunction() completes, its scope chain is destroyed but its activation object will
remain in memory until the anonymous function is destroyed, as in the following:
//create function
var compareNames = createComparisonFunction(“name”);
//call function
var result = compareNames({ name: “Nicholas” }, { name: “Greg”});
//dereference function - memory can now be reclaimed
compareNames = null;
Here, the comparison function is created and stored in the variable compareNames . Setting
compareNames equal to null dereferences the function and allows the garbage collection routine to
clean it up. The scope chain will then be destroyed and all of the scopes (except the global scope) can be
destroyed safely. Figure 7 - 2 shows the scope - chain relationships that occur when compareNames() is
called in this example.
createComparisonFunction
[[Scope]]
Scope Chain
1
0
Global Scope
this
createComparisonFunction
result
window
undefined
(anonymous)
[[Scope]]
Figure 7 - 2
Scope Chain
2
1
0
createComparisonFunction Scope
this
window
arguments
["name"]
propertyName
"name"
(anonymous) Scope
this
window
arguments
[]
object1
undefined
object2
undefined
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