How would defining `that = this` help me create private variables/members?

Question

I was reading Douglas Crawford's piece on creating private variables in javascript classes.

In it he says you have to state that = this in order to "make the object available to private methods". However, I was able to build an example which has private members, private methods and public methods without defining that = this:

function Form(id_code) {

    //private variable
    var id_code = id_code;
    var color = '#ccc';

    //private method
    function build_style_attribute() {
        return 'style="background-color:'+color+'"';
    }

    //public method
    this.render = function() {
        return '<div '+build_style_attribute()+'>'+id_code+'</div>';
    }
}

var formModules = new Form('modules');

$('p#test').html(formModules.render());

What would specifying that = this allow me to do which this example does not already do?

Added:

Thanks @Gaby, so this is how I understand it: as the above example shows, I have access to private variables without using that=this but it does give me access to public variables as shown here:

function Form(id_code) {
    that = this;

    //private variable
    var id_code = id_code;
    var color = '#ccc';

    //public variable
    this.weight = 'bold';

    //private method
    function build_style_attribute() {
        //this will not work with either "weight" or "this.weight"
        return 'style="background-color:'+color+'; font-weight:'+that.weight+'"';
    }

    //public method
    this.render = function() {
        return '<div '+build_style_attribute()+'>'+id_code+'</div>';
    }
}

var formModules = new Form('modules');

$('p#test').html(formModules.render());

Answer 1

By convention, we make a private that variable. This is used to make the object available to the private methods.

This is a workaround for an error in the ECMAScript Language Specification which causes this to be set incorrectly for inner functions.

function Test() {
    var that = this;
    
    function wrongprivate(){
     return this;
    }
    
    function rightprivate(){
     return that;
    }    
    
    this.check= function (){
     console.log( wrongprivate() );
     console.log( rightprivate() );
    }
    
}

var test= new Test();
test.check();
// will output 
// window
// object{}

Live at http://www.jsfiddle.net/BpmQ3/1/