My intuition is that it's a good idea to encapsulate blocks of code in anonymous functions like this:

(function() {
  var aVar;
  aVar.func = function() { alert('ronk'); };
  aVar.mem = 5;
})();

Because I'm not going to need aVar again, so I assume that the garbage collector will then delete aVar when it goes out of scope. Is this right? Or are interpreters smart enough to see that I don't use the variable again and clean it up immediately? Are there any reasons such as style or readability that I should not use anonymous functions this way?

Also, if I name the function, like this:

var operations = function() {
  var aVar;
  aVar.func = function() { alert('ronk'); };
  aVar.mem = 5;
};
operations();

does operations then necessarily stick around until it goes out of scope? Or can the interpreter immediately tell when it's no longer needed?

A Better Example

I'd also like to clarify that I'm not necessarily talking about global scope. Consider a block that looks like

(function() {

  var date = new Date(); // I want to keep this around indefinitely

  // And even thought date is private, it will be accessible via this HTML node
  // to other scripts.
  document.getElementById('someNode').date = date;

  // This function is private
  function someFunction() {
    var someFuncMember;
  }

  // I can still call this because I named it. someFunction remains available.
  // It has a someFuncMember that is instantiated whenever someFunction is
  // called, but then goes out of scope and is deleted. 
  someFunction();

  // This function is anonymous, and its members should go out of scope and be
  // deleted
  (function() {
    var member;
  })(); // member is immediately deleted
  // ...and the function is also deleted, right? Because I never assigned it to a
  // variable. So for performance, this is preferrable to the someFunction
  // example as long as I don't need to call the code again.

})();

Are my assumptions and conclusions in there correct? Whenever I'm not going to reuse a block, I should not only encapsulate it in a function, but encapsulate it in an anonymous function so that the function has no references and is deleted after it's called, right?

---

You're right that sticking variables inside an anonymous function is a good practice to avoid cluttering up the global object.

To answer your latter two questions: It's completely impossible for the interpreter to know that an object won't be used again as long as there's a globally visible reference to it. For all the interpreter knows, you could eval some code that depends on window['aVar'] or window['operation'] at any moment.

Essentially, remember two things:

As long as an object is around, none of its slots will be magically freed without your say-so.

Variables declared in the global context are slots of the global object ( window in client-side Javascript).

Combined, these mean that objects in global variables last for the lifetime of your script (unless the variable is reassigned). This is why we declare anonymous functions — the variables get a new context object that disappears as soon as the function finishes execution. In addition to the efficiency wins, it also reduces the chance of name collisions.

Your second example (with the inner anonymous function) might be a little overzealous, though. I wouldn't worry about "helping the garbage collector" there — GC probably isn't going to run in the middle that function anyway. Worry about things that will be kept around persistently, not just slightly longer than they otherwise would be. These self-executing anonymous functions are basically modules of code that naturally belong together, so a good guide is to think about whether that describes what you're doing.

There are reasons to use anonymous functions inside anonymous functions, though. For example, in this case:

(function () {
  var bfa = new Array(24 * 1024*1024);
  var calculation = calculationFor(bfa);
  $('.resultShowButton').click( function () {
    var text = "Result is " + eval(calculation);
    alert(text);
  } );
})();

This results in that gigantic array being captured by the click callback so that it never goes away. You could avoid this by quarantining the array inside its own function.

---

Anything that you add to the global scope will stay there until the page is unloaded (unless you specifically remove it).

It's generally a good idea to put variables and function that belong together either in a local scope or in an object, so that they add as little as possible to the global namespace. That way it's a lot easier to reuse code, as you can combine different scripts in a page with minimal risks for naming collisions.

链接地址: http://www.djcxy.com/p/51494.html

上一篇: Scala关闭如何转换为Java对象？

下一篇: 我应该在匿名JavaScript函数中封装功能块吗？