How to maintain JavaScript function variable states (values) between calls?

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?

Are you missing a closing parenthesis

mowwwalker
– mowwwalker

2011-10-02 08:55:53 +00:00
Commented Oct 2, 2011 at 8:55 — mowwwalker
– mowwwalker, Commented Oct 2, 2011 at 8:55

Rob W · Accepted Answer · 2011-10-02 08:58:21Z

4

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.

edited Oct 2, 2011 at 8:58

answered Oct 2, 2011 at 8:56

Rob W

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Comments

ThiefMaster · Accepted Answer · 2011-10-02 08:52:33Z

4

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 (e.g. this._val).

answered Oct 2, 2011 at 8:52

ThiefMaster

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Comments

karnyj · Accepted Answer · 2011-10-02 09:01:00Z

4

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.
  }
})();

edited Oct 2, 2011 at 9:01

answered Oct 2, 2011 at 8:55

karnyj

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