What is JavaScript?

✒ Reference: Introduction - JavaScript by Mozilla

What is JavaScript?

• A cross-platform, object-oriented scripting language to make webpages interactive
• Connects the objects of its environment to provide programmatic control inside a host environment.
• Core JS can be extended by supplementing it with additional objects.
  • Client-side JS: extends the core language by supplying objects to control a browser and its Document Object Model (i.e. respond to user events).
  • Server-side JS: extends the core language by supplying objects relevant to running JS on a server (i.e. communicate with a DB).

JS and Java

• JS follows most Java expression syntax, naming conventions, and basic control-flow constructs.
• Java
  • compile-time system of classes built by declarations
• JS
  • Runtime system based on smaller sets of data types
  • Prototype-based object model (dynamic inheritance: what is inherited can vary for individual objects)
  • free-form language: dynamic; variables, parameters, function return types are not explicitly typed.

JavaScript	Java
Object-oriented. No distinction between types of objects. Inheritance is through the prototype mechanism, and properties and methods can be added to any object dynamically.	Class-based. Objects are divided into classes and instances with all inheritance through the class hierarchy. Classes and instances cannot have properties or methods added dynamically.
Variable data types are not declared (dynamic typing, loosely typed).	Variable data types must be declared (static typing, strongly types).
Cannot automatically write to hard disk.	Can automatically write to hard disk.

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Getting started with JavaScript

• Web Console (Ctrl+Shift+I): returns the last expression entered

• Every time something is entered into the console, it is actually surrounded by console.log around eval

console.log(eval('3 + 5'))

• My first “Hello world” JS code!

(function(){
"use strict";
/* Start of your code */
function greetMe(yourName) {
alert('Hello ' + yourName);
}

greetMe('World');
/* End of your code */
})();

• (function(){"use strict"; prevents semantics in JavaScript, and prevents code snippets executed in the console from interacting with one-another