Objects in Dynamically Typed Languages like JavaScript

CS 301: Assembly Language Programming Lecture, Dr. Lawlor

First, let's address the the biggest misconception about JavaScript:

Java is a statically-typed, compiled language that runs in a standardized cross-platform virtual machine. Created at Sun Microsystems, now owned by Oracle, it's the direct inspiration for Microsoft's C# and the Android Dalvik VM. It's still hanging on in backend servers, but on the web it has nearly died out out due to poor browser plugin support and recurring security problems.
JavaScript is a dynamically-typed, interpreted language that runs in a diverse variety of web browsers. Created at Netscape around the same time as Java, in 1995, it has vaguely similar syntax and garbage collection, but virtually no other similarities. JavaScript is now the dominant way to program client-side web pages, Flash games (ActionScript), and even web servers (node.js). Many translators use it as a backend, including emscripten, asm.js, CoffeeScript, etc. It has been standardized by the EMCA as ECMAScript, although this is an unwieldy name.

The cool part about JavaScript is you can embed interactive stuff in web pages, like this:
Code Demo Page on jsbin.com

The biggest difference between JavaScript and C++ is JavaScript uses dynamic typing--you declare any kind of variable with "var". At runtime, you can ask what a variable's type is using "typeof", which will return one of these:

JavaScript typeof	Example	C++ type (roughly)
number	var x=3;	int, float, double
string	var x="bob";	std::string, char *
boolean	var x=false;	boolean
function	var x=function() {...};	any function, method
object	var x={"y":3};	any class, struct, or map!
object	var x=[2,3,5];	std::vector, array
undefined	var x;	void

Javascript doesn't even force you to declare variables--any newly encountered variable is treated as having the value "undefined". This can be annoying, because when you typo a variable name, you don't get a compile error, only a crash at runtime ("Uncaught TypeError: Cannot call method of undefined").

Javascript objects are also very surprisingly flexible--you can add new methods and fields to any object at any time!
Code Demo Page on jsbin.com

That JSON.stringify converts an object to a string representation in JavaScript Object Notation, an increasingly common data format.

Internally, a JSON object is stored in a dynamically allocated data structure similar to a std::map<std::string, value *> (for example, see this C++ implementation of JSON objects). Because dynamic allocation is expensive, high performance libraries like the 3D rendering package three.js have an incentive to re-use objects.