javascript instantiation and arguments.callee

最新推荐文章于 2017-11-25 17:39:32 发布

iteye_13882

最新推荐文章于 2017-11-25 17:39:32 发布

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分类专栏： javascript 文章标签： javascript ViewUI

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本文详细解释了构造函数与普通函数的区别，并提供了防止误调用构造函数导致意外覆盖变量值的方法。通过实例演示了如何利用 arguments.callee 巧妙地确保构造函数被正确调用。

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in the nutshell, it is no difference between a constructor and function, because a constructor is essentially a function.

however, if you just give user a raw constructor, there is no way for the user to mistakenly call it like a normal function. and the consequence is sometimes horrible.

e.g

function User(first, last){
  this.name = first + " " + last;
}
var name = "Resig";
var user = User("John", name);
assert( name == "John Resig", "The name variable is accidentally overridden." );

How to avoid such kind of thing, here is one deal.

/**************************************
*@Summary
* Make sure that the constructor is called in the right way 
*
* Make use of the arguments.callee trick; After this trick, you can find the User function behave like a constructor and a static constructor at the same time.
*
* @Usage:
*   

var name = "Resig";
var user = User1("John", name);
assert( name == "John Resig",
"The name variable is accidentally overridden." );


now, what you can do is to do this :

var name = "Resig";
var user = User("John", name);
assert(user, "This was defined correctly,even if it was by mistake");

* @TODO:
* test it 
***************************************/


function User1(first, second) {
  this.name = first + " " + second;
}


function User(first, second) {

  if (!(this  instanceof arguments.callee)) {
    return new User(first, second);
  }

  this.name = first + " " + second;
}

the statement, 'this instanceof arguments.callee will test if the 'this' is of the type of the callee, in this case, "User", and if it is, just run the constructor as normal (the else part), however, if 'this' is not the 'User', it will instead call and return the constructor.

below is the html script that test the function. HTH.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
    <script src="instantiation.js" type="text/javascript"></script>
    <script src="../unit.js" type="text/javascript"></script>
    <script type="text/javascript">
      window.onload = function () {
        test("test the instantiation of the function", function () {
          var name = "Resig";
          var user = User("John", name);

          assert(user, "This was defined correctly,even if it was by mistake");
        });
      };
    </script>
    <style type="text/css">
      #results li.pass { color: Green }
      #results li.fail { color: Red }
    </style>
</head>
<body>
<ul id="results" />
</body>
</html>