模块模式中的私有变量

Question

I am not clear about the concept of private, if I can still access it through the public method and redefine the properties of the module. I mean, I can perfectly do:

var aModule = (function() {

    var privateVar = 1;

    return {
        publicFunction: function() {
            return privateVar;
        }
    }

})();

aModule.publicFunction = function() {
    privateVar = function() {
        console.log('a new content');
    }
    privateVar();
};

aModule.publicFunction(); // 'a new content'

I understand that this is not possible if I write it in ES6 with let or const, because it would give me error try to overwrite the value of the private variable, but what sense does it have in ES5?

Answer 1

A private variable cannot be accessed or changed by code that's outside the module or class that owns the private variable.

For example, you can't do aModule.privateVar and expect it to give you anything back.

What your publicFunction is is what the Java (and other programming languages) world would call a "getter". Simply put it gives access to the value of the private variable, without allowing write access to it.

In your last example, you're not actually overwriting the private variable. You're just creating a new variable within publicFunction 's scope and assigning a value to that. Just because it's also named privateVar doesn't mean it's the same area of memory.

I've added to your code to demonstrate this

 var aModule = (function() { var privateVar = 1; return { publicFunction: function() { return privateVar; }, getPrivateVar() { return privateVar; } } })(); aModule.publicFunction = function() { privateVar = function() { console.log('a new content'); } privateVar(); }; aModule.publicFunction(); // 'a new content' console.log(aModule.getPrivateVar()); //outputs 1 

To give more detail on why this is, it's all about scope. privateVar exists in an anonymous function's scope. This anonymous function returns an object with several functions defined on it. When the function is called, this object is assigned to aModule, but retains access to privateVar because they share scope.

However, outside of that function we're at a different scope, which doesn't have access to the aModule's variables, except those exposed in the returned object

Answer 2

You are overwriting the public function and not able to access the module private variable

Consider the following new function that is created as a property of aModule that attempts to only change the value of var privateVar

The scope is different because of where it gets called and it isn't able to access that private variable

 var aModule = (function() { var privateVar = 1; return { publicFunction: function() { return privateVar; } } })(); aModule.newFunction = function() { // try to change privateVar in aModule privateVar = 3 }; aModule.newFunction(); console.log(aModule.publicFunction()); //still 1 not 3 // here's where it actually ended up console.log(window.privateVar)