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JavaScript UP Access

第5版真实在

JavaScript functions are a combination of code to be executed and the scope in which to execute them. This combination of code and scope is known as a closure in the computer science literature. All JavaScript functions are closures. These closures are only interesting, however, in the case discussed above: when a nested function is exported outside the scope in which it is defined. When a nested function is used in this way, it is often explicitly called a closure.

When the JavaScript interpreter invokes a function, it first sets the scope to the scope chain that was in effect when the function was defined. Next, it adds a new object, known as the call object (the ECMAScript specification uses the term activation object) to the front of the scope chain. The call object is initialized with a property named arguments that refers to the Arguments object for the function. Named parameters of the function are added to the call object next. Any local variables declared with the var statement are also defined within this object. Since this call object is at the head of the scope chain, local variables, function parameters, and the Arguments object are all in scope within the function. This also means that they hide any properties with the same name that are further up the scope chain, of course.

When no nested functions are involved, the scope chain is the only reference to the call object. When the object is removed from the chain, there are no more references to it, and it ends up being garbage collected.

But nested functions change the picture. If a nested function is created, the definition of that function refers to the call objects because that call object is the top of the scope chain in which the function was defined. If the nested function is used only within the outer function, however, the only reference to the nested function is in the call object. When the outer function returns, the nested function refers to the call object, and the call object refers to nested function, but there are no other references to either one, and so both objects become available for garbage collection.

Things are different if you save a reference to the nested function in the global scope. You do so by using the nested function as the return value of the outer function or by storing the nested function as the property of some other object. In this case, there is an external reference to the nested function, and the nested function retains its reference to the call object of the outer function. The upshot is that the call object for that one particular invocation of the outer function continues to live, and the names and values of the function arguments and local variables persist in this object. JavaScript code cannot directly access the call object in any way, but the properties it defines are part of the scope chain for any invocation of the nested function. (Note that if an outer function stores global references to two nested functions, these two nested functions share the same call object, and changes made by an invocation of one function are visible to invocations of the other function.)

The object through which a method is invoked becomes the value of the this keyword within the body of the method.

While it is useful to think of functions and methods differently, there is not actually as much difference between them as there initially appears to be. Recall that functions are values stored in variables and that variables are nothing more than properties of a global object. Thus, when you invoke a function, you are actually invoking a method of the global object. Within such a function, the this keyword refers to the global object. Thus, there is no technical difference between functions and methods. The real difference lies in design and intent: methods are written to operate somehow on the this object, while functions usually stand alone and do not use the this object.

When a function is invoked as a function rather that as a method, the this keyword refers to the global object. Confusingly, this is true even when a nested function is invoked (as a function) within a containing method that was invoked as a method: the this keyword has one value in the containing function but (counterintuitively) refers to the global object within the body of the nested function.

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