如何在调用之间维护JavaScript函数变量状态（值）？

Question

I am looking for getters and setters functionality but cannot rely on __defineGetter__ and __defineSetter__ yet. So how does one maintain a function variable's value between function calls?

I tried the obvious, but myvar is always undefined at the start of the function:

FNS.itemCache = function(val) {
    var myvar;
    if( !$.isArray(myvar)
        myvar = [];
    if( val === undefined)
        return myvar;
    .. // Other stuff that copies the array elements from one to another without
       // recreating the array itself.
};

I could always put another FNS._itemCache = [] just above the function, but is there a way to encapsulate the values in the function between calls?

Answer 1

You can store the value on the function by using arguments.callee as a reference to the current function:

FNS.itemCache = function(val) {
    if( !$.isArray(arguments.callee._val)
        arguments.callee._val = [];
    if(val === undefined)
        return arguments.callee._val;
    .. // Other stuff that copies the array elements from one to another without
       // recreating the array itself.
};

However, this will break if the function is stored in a prototype and thus used by more than one object. In this case you have to use a member variable (eg this._val ).

Answer 2

this is a standard pattern for creating your static variable and for creating private members of an object

FNS.itemCache = (function() {
  var myvar;
  if( !$.isArray(myvar)
      myvar = [];
  return function(val) {
      if( val === undefined)
          return myvar;
         .. // Other stuff that copies the array elements from one to another without
         // recreating the array itself.
  }
})();

Answer 3

An alternative way to set a private variable is by wrapping the function definition in an anonymous function:

(function(){
    var myvar;
    FNS.itemCache = function(val) {
        if( !$.isArray(myvar))
            myvar = [];
        if( typeof val == "undefined")
            return myvar;
        .. // Other stuff that copies the array elements from one to another without
           // recreating the array itself.
    };
})();

This way, myvar is defined in the scope of FNS.itemCache . Because of the anonymous function wrapper, the variable cannot be modified from elsewhere.