Private variable not staying private in JS module

Question

After reading the following article, http://javascriptplayground.com/blog/2012/04/javascript-module-pattern/ I have decided to start implementing modules in my JS.

Unfortunately, the module I am using does not seem to be keeping the private variable private,

var popoverOptionsModule = (function() {
var _stopAskingList = [];

var addToStopAskingList = function(itemToAdd) {
    if (_stopAskingList.indexOf(itemToAdd) === -1){
      _stopAskingList.push(itemToAdd);
    }
}

var getStopAskingList = function() {
    return _stopAskingList;
}


return {
    addToStopAskingList: addToStopAskingList,
    getStopAskingList: getStopAskingList,
};
})();

popoverOptionsModule._stopAskingList = 4;
console.log(popoverOptionsModule._stopAskingList);

As you can see, I am able to change the value of popoverOptionsModule._stopAskingList and log the update to the console... I thought this was not supposed to happen. Thanks for your help!

Answer 1

JS is completely dynamically typed, so when you have the line

popoverOptionsModule._stopAskingList = 4;

You've just created this variable and assigned it a value, hence why the next line succeeds. If you didn't have this line, then the subsequent console.log would report undefined. This code would work too

popoverOptionsModule._abc = 4;
console.log(popoverOptionsModule._abc);

Remember that this isn't actually a private variable in the same way that OO languages implement protection levels, rather it's just an API pattern that attempts to hide it from the caller.