How does function scope work in JavaScript? -

i having trouble understanding function scope in javascript:

function person() {     var _firstname;     var _lastname; } personone = new person(); personone._firstname = "fred"; alert(personone._firstname);   

this outputs "fred", thought variables of person function accessible inside function. why work?

in javascript, objects dynamically-expandable.

for example:

var obj = {}; obj.firstname = "matías"; // <-- adds property if doesn't exist 

in other hand, if want declare properties should part of object in constructor function need qualify them this:

function person() {     this._firstname = null;     this._lastname = null; } 

extra info

if want avoid objects being dynamically-expandable, can use ecma-script 5 object.preventextensions function:

var obj = {}; object.preventextensions(obj);  // property won't added! obj.text = "hello world";