javascript private function access public variable

Functions are not tied to instances, therefore your invocation of func2 ends up as invocation without this pointing to the expected instance.

You can either fix the invocation to include the context:

function ctest() {
    this.var1 = "haha";
    this.func1 = function() {
        alert(this.var1);
        func2.call(this);
        alert(this.var1);
    }
    var func2 = function() {
        this.var1 = "huhu";
    }
}

Or you can keep a variable with the wanted object reference around:

function ctest() {
    var that = this;
    this.var1 = "haha";
    this.func1 = function() {
        alert(this.var1);
        func2();
        alert(this.var1);
    }
    var func2 = function() {
        that.var1 = "huhu";
    }
}

Another option is to use Function.bind:

function ctest() {
    this.var1 = "haha";
    this.func1 = function() {
        alert(this.var1);
        func2();
        alert(this.var1);
    }
    var func2 = function() {
        this.var1 = "huhu";
    }.bind(this);
}

Note that the browser support for this is not perfect.

There is no such thing as private in JS, but you can play with scopes using the closures.

Let's say for instance that in your example you don't need to expose var1 as public property. You could easily rewrite your code as the follows:

function ctest() {
    var var1 = "haha";

    this.func1 = function() {
        alert(var1);
        func2();
        alert(var1);
    }

    var func2 = function() {
        var1 = "huhu";
    }
}

Because both func1 and func2 shares the same scope – they are defined in the same function, ctest – they can access to the same variables. Of course in that case you don't have var1 exposed, so: myobj.var1 will be undefined.

If you want var1 to be exposed as property, then what you need is bind func2 to the object instance you created in the constructor:

function ctest() {
    this.var1 = "haha";
    this.func1 = function() {
        alert(this.var1);
        func2();
        alert(this.var1);
    }
    var func2 = function() {
        this.var1 = "huhu";
    }.bind(this);
}

Otherwise func2 will have a different context object (this). If the browser doesn't support bind and you don't want use a shim (as shown in the link above), you can take advantage of the closures again:

function ctest() {
    this.var1 = "haha";
    this.func1 = function() {
        alert(this.var1);
        func2();
        alert(this.var1);
    }
    var context = this;
    var func2 = function() {
        context.var1 = "huhu";
    }
}

IMVHO is less elegant.

function ctest() {
    this.var1 = "haha";
    this.func1 = function() {
        alert(this.var1);
        func2(this);
        alert(this.var1);
    }
    var func2 = function(obj) {
        obj.var1 = "huhu";
    }
}

And there is other way to solve this problem, check the other answers :)