Question

I am trying to figure out how to have a private instance variable on my class that gets set by a parameter of a public method. In this case, though; it seems that outside of somePublicMethod myPrivateVar will be undefined. How can achieve what I am trying to do ?

MyClass = function() {
    var myPrivateVar;

    this.somePublicMethod(myPrivateVar) {
        myPrivateVar = myPrivateVar //????
    }

    this.someOtherPublicMethod() {
        somePrivateMethod();
    }

    function somePrivateMethod() {
        myPrivateVar++;
    }
}

Answer 1

The issue is you’re shadowing the var myPrivateVar by giving the argument the same name, so only the argument variable is in scope:

this.somePublicMethod = function(myPrivateVar) {
    myPrivateVar = myPrivateVar; // sets the argument to itself
}

You’ll need to give one of them a different name to avoid shadowing:

this.somePublicMethod = function(inputVar) {
    myPrivateVar = inputVar;
};

Otherwise, you’ll need to contain one of them somehow:

MyClass = function () {
    var locals = {
        myPrivateVar: null
    };

    this.somePublicMethod = function (myPrivateVar) {
        locals.myPrivateVar = myPrivateVar;
    };

    function somePrivateMethod() {
        locals.myPrivateVar++;
    }
};