javacript getter/setter

http://ejohn.org/blog/javascript-getters-and-setters/

JavaScript Getters and Setters

It is with much happiness that I think I can finally say, without seeming like a fool, that: "JavaScript Getters and Setters are now prevalent enough to become of actual interest to JavaScript developers." Wow, I've been waiting a long time to be able to say that.

I want to start by giving a whirlwind tour of Getters and Setters and why they're useful. Followed by a look into what platforms now support Getters and Setters as to make them relevant.

Getters and Setters

Getters and Setters allow you to build useful shortcuts for accessing and mutating data within an object. Generally, this can be seen as an alternative to having two functions with an object that are used to get and set a value, like so:

{
    getValue:  function ( ) {
         return  this._value;
     },
    setValue:  function (val ) {
         this._value = val;
     }
}

The obvious advantage to writing JavaScript in this manner is that you can use it obscure values that you don't want the user to directly access. A final result looking something like the following (using a closure to store the value of a newly constructed Field):

function Field (val ) {
     var value = val;
    
     this. getValue =  function ( ) {
         return value;
     };
    
     this. setValue =  function (val ) {
        value = val;
     };
}

Some example results:

var field =  new Field ( "test" );
field. value
// => undefined
field. setValue ( "test2" )
field. getValue ( )
// => "test2" 
 

Now, centered around this concept, is where getters and setters come into play. They allow you to bind special functions to an object that look like normal object properties, but actually execute hidden functions instead. The end result looks something like this:

var field =  new Field ( "test" );
field. value
// => test
field. value =  "test2";
field. value
// => "test2" 
 

So let's look at how you would go about setting something like that up. Mimicking the "hidden value property" style of before, our code would look something like this:

function Field (val ) {
     var value = val;
    
     this.__defineGetter__ ( "value",  function ( ) {
         return value;
     } );
    
     this.__defineSetter__ ( "value",  function (val ) {
        value = val;
     } );
}

Now, if we wanted to, instead, define getters and setters within the context of our object prototype (and where having "private" data is less of a concern) we can then use an alternative object syntax for that.

function Field (val ) {
     this. value = val;
}

Field.prototype = {
    get value(){
        return this._value;
    },
    set value(val){
        this._value = val;
    }
};

The syntax for getters and setters is typically what scare people the most about the feature. But after a little bit of use, it's easy to get over.

Here's another example, allowing a user to access an array of usernames (but denying them access to the original, underlying user objects.

function Site (users ) {
     this.__defineGetter__ ( "users",  function ( ) {
         // JS 1.6 Array map()
         return users. map ( function (user ) {
             return user. name;
         } );
     } );
}

As a bonus, here's a method that I've written that can help you to extend one object with another (a common JavaScript operation) while still taking into account getters and setters:

// Helper method for extending one object with another
function extend (a,b )  {
     for  (  var i  in b  )  {
         var g = b.__lookupGetter__ (i ), s = b.__lookupSetter__ (i );
       
         if  ( g || s  )  {
             if  ( g  )
                a.__defineGetter__ (i, g );
             if  ( s  )
                a.__defineSetter__ (i, s );
          }  else
             a [i ] = b [i ];
     }
     return a;
}

This code is from the Server-Side Browser Environment that I wrote about a week ago. You'll notice that, when looking through the code that I make liberal use of getters and setters to create this mock environment.

Additionally, in my custom extend() method you'll notice two new methods: __lookupGetter__ and __lookupSetter__. These are immensely useful, once you start dealing with getters and setters.

For example, when I did my first pass at writing an extend() method, I started getting all sorts of errors - I was thoroughly confused. That's when I realized that two things were happening with the simple statement: a[i] = b[i];

If a setter existed in object a, named i, and a getter existed in object b, named i, a[i]'s value was being set not to the other setter function, but to the computed value from b's getter function. The two __lookup*__ methods allow you to access the original functions used for the methods (thus allowing you to write an effective extend method, for example).

A couple things to remember:

• You can only have one getter or setter per name, on an object. (So you can have both one value getter and one value setter, but not two 'value' getters.)
• The only way to delete a getter or setter is to do: 'delete object[name];' Be aware, that this command is capable of deleting normal properties, getters and setters. (So if all you want to do is remove a setter you need to backup the getter and restore it afterwards.)
• If you use __defineGetter__ or __defineSetter__ it will silently overwrite any previous named getter or setter - or even property - of the same name.

Platforms

Now, here is where things get really interesting. Up until just recently, talking about actually using getters and setters (outside of the context of Firefox/Gecko) was pretty much not happening.

However, look at this line up:

Browsers

• Firefox
• Safari 3+ (Brand New)
• Opera 9.5 (Coming Soon)

I used the following snippet to test browsers:

javascript:foo= {get test ( ) {  return  "foo";  } };alert (foo. test );

Additionally, the following JavaScript environments support Getters and Setters

• SpiderMonkey
• Rhino 1.6R6 (New)

And as do the following ECMAScript implementations

• ActionScript
• JScript.NET 8.0

Posted: July 18th, 2007 ·

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