What is the point of undefined variable in javascript module pattern

Question

I come across the following javascript module pattern and like it very much, but, why the use of undefined in the argument?

(function(window, document, undefined){

    'use strict';

    var MyWidget = function(){

        if (!(this instanceof MyWidget)) {
            var test = new MyWidget();
            return test.init.call(test, Array.prototype.slice.call(arguments));
        }

      var firstPrivateVar = 'aa',
          secondPrivateVar = 'bb';

      this.init = function(options){
        console.log('MyWidget.init', options);
        return true;
      };

      this.setStuff = function(){

      };

    };

    window.MyWidget = MyWidget;

})(window, document);

why pass in window and document?? are they not always accessible? — olefrank, Jul 31 '15 at 12:57

score 2 · Answer 1 · answered Sep 12 '14 at 06:17

In ECMAScript 3 the developer had the right to use the undefined keyword as a variable i.e. it was mutable. So if I were to write

undefined = true;

the value of undefined would be true and it would lose its true meaning. In this above mentioned scenario, we are passing the window object, the document object and there is no third parameter. so this means that in function(window, document, undefined), the undefined variable will be actually undefined as we are not passing any parameter to it.

According to ECMAScript5, this undefined in not mutable any more

score 1 · Answer 2 · answered Sep 12 '14 at 06:14

1

undefined can be overwritten in some browsers. This fixes the problem in your scope by redefining undefined to the value of a non-existent parameter.

answered Sep 12 '14 at 06:14

Rob M.

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What is the point of undefined variable in javascript module pattern

2 Answers2