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return base + num;
Listing 5.17 uses adder to create incrementing and decrementing functions.
Listing 5.17 Incrementing and decrementing functions
TestCase("AdderTest", {
"test should add or subtract one from arg": function () {
var inc = adder(1);
var dec = adder(-1);
assertEquals(3, inc(2));
assertEquals(3, dec(4));
assertEquals(3, inc(dec(3)));
The scope chain for the inc method contains its own activation object at the
front. This object has a num property, corresponding to the formal parameter. The
base variable, however, is not found on this activation object. When JavaScript
does identifier resolution, it climbs the scope chain until it has no more objects.
When base is not found, the next object in the scope chain is tried. The next
object is the activation object created for adder , where in fact the base property
is found. Had the property not been available here, identifier resolution would have
continued on the next object in the scope chain, which in this case is the global
object. If the identifier is not found on the global object, a reference error is thrown.
Inside the functions created and returned from adder , the base variable is
known as a free variable , which may live on after the adder function has finished
executing. This behavior is also known as a closure , a concept we will dig deeper
into in the next chapter, Chapter 6, Applied Functions and Closures.
Functions created by the Function constructor have different scoping rules.
Regardless of where they are created, these functions only have the global object in
their scope chain, i.e., the containing scope is not added to their scope chain. This
makes the Function constructor useful to avoid unintentional closures.
5.3.6 Function Expressions Revisited
With a better understanding of the scope chain we can revisit function expressions
and gain a better understanding of how they work. Function expressions can be use-
ful when we need to conditionally define a function, because function declarations
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