显示模块模式和范围

Question

I ran into an issue with some javascript I was running and broke it down to the code below. I'm confused as to why the _localVar variable doesn't change after init() is called. I find that if I reference this._localVar in the revealingModule, the results are as expected. I am confused about the scope. Could someone please clarify why this is occurring. I thought that if I didn't use this , then the next scope would be the module, but I don't think that's happening.

var myRevealingModule = (function () {
    var _localVar = "Default";

    function init() {
        console.log(_localVar);
        _localVar = "Init";
    }

    function getTest() {
        console.log(_localVar);
    }

    return {
        init: init,
        getTest: getTest,
        localVar: _localVar
    };
})();

myRevealingModule.getTest();               // "Default"
console.log(myRevealingModule.localVar);   // "Default"
myRevealingModule.init();                  // "Default"
myRevealingModule.getTest();               // "Init"
console.log(myRevealingModule.localVar);   // "Default"   * WHY *

Answer 1

myRevealingModule.localVar is not a reference to the variable's value; it simply copied the string when it's created.

When you use this.localVar , you're using the variable in the returned object. Hence, when you change that identifier, it also updates with myRevealingModule.localVar .

Answer 2

Note that your module uses a self-invoking function. Therefore the value of myRevealingModule.localVar is determined right after the definition and built-in invocation of myRevealingModule . At this time the value of _localVar is "Default", which is copied to returned object's localVar property. Even if you change _localVar afterwards this has no effect on myRevealingModule.localVar anymore.

Basically the following example show the same effect:

var a = 42;
var b = a;
console.log(a); // 42
console.log(b); // 42
a = 84;
console.log(a); // 84
console.log(b); // 42

b copies the value of a . If you change a afterwards that has no impact on b .