Short illustration of what `this` means in various scopes, when using a constructor function approach.

WhatIsThis.js

var WhatIsThis = function () {
    var _this = this; // `_this` or `that` or `self`, take your pick

    // References to Window below assume you're running in a browser and not node.js

    console.log("'this' directly in constructor body: ", this); // 'this' is WhatIsThis

    function privateFunc() {
        console.log("'this' in privateFunc: ", this); // 'this' is Window
        console.log("'_this' in privateFunc: ", _this); // '_this' is WhatIsThis
    }

    this.publicFunc = function () {
        console.log("'this' in publicFunc: ", this); // 'this' is WhatIsThis
        console.log("'_this' in publicFunc: ", _this); // '_this' is WhatIsThis
        console.log("calling private functions from public function");
        privateFunc();
        alsoPrivateFunc();
    };

    var alsoPrivateFunc = function () {
        console.log("'this' in alsoPrivateFunc: ", this); // 'this' is Window
        console.log("'_this' in alsoPrivateFunc: ", _this); // '_this' is WhatIsThis
    };

    console.log("calling private functions from constructor");
    privateFunc();
    alsoPrivateFunc();
};

WhatIsThis.prototype.otherPublicFunc = function () {
    console.log("'this' in otherPublicFunc: ", this); // 'this' is WhatIsThis
    // cannot call functions or reference variables that are private to the
    // constructor function scope here.
    //console.log("'_this' in otherPublicFunc: ", _this); // ERROR
    //privateFunc(); // ERROR
    //alsoPrivateFunc(); // ERROR
};

var inst = new WhatIsThis();
inst.publicFunc();
inst.otherPublicFunc();