javascript

Chapter 7: Anonymous Functions
a closure that accesses num is created and returned. Now each function in the result array has its own
copy of num , and thus can return separate numbers.
The this Object
Using the this object inside closures introduces some problems. The this object is bound at runtime
based on the context in which a function is executed: when used inside global functions, this is equal to
window , whereas this is equal to the object when called as an object method. Anonymous functions are
considered to be global in this context, so the this object always points to window . Due to the way
closures are written, however, this fact is not always obvious. Consider the following:
var name = “The Window”;
var object = {
name : “My Object”,
};
getNameFunc : function(){
return function(){
return this.name;
};
}
alert(object.getNameFunc()()); //”The Window”
Here, a global variable called name is created along with an object that also contains a property called
name . The object contains a method, getNameFunc() , that returns an anonymous function, which
returns this.name . Since getNameFunc() returns a function, calling object.getNameFunc()()
immediately calls the function that is returned, which returns a string. In this case, however, it returns
“ The Window ” , which is the value of the global name variable. Why didn ’ t the anonymous function pick
up the containing scope ’ s this object?
Remember that each function automatically gets two special variables in its activation object as soon as
the function is called: this and arguments . An inner function can never access these variables from an
outer function directly since the search for variables with these names stops on the inner function ’ s
activation object (look back at Figure 7 - 2 for more information). It is possible to allow a closure access to
a different this object by storing it in another variable that the closure can access, as in this example:
var name = “The Window”;
var object = {
name : “My Object”,
};
getNameFunc : function(){
var that = this;
return function(){
return that.name;
};
}
alert(object.getNameFunc()()); //”My Object”
189