JavaScript and Java

JavaScript and Java are similar in some ways but fundamentally different in some others. The JavaScript language resembles Java but does not have Java's static typing and strong type checking. JavaScript follows most Java expression syntax, naming conventions and basic control-flow constructs which was the reason why it was renamed from LiveScript to JavaScript.

In contrast to Java's compile-time system of classes built by declarations, JavaScript supports a runtime system based on a small number of data types representing numeric, Boolean, and string values. JavaScript has a prototype-based object model instead of the more common class-based object model. The prototype-based model provides dynamic inheritance; that is, what is inherited can vary for individual objects. JavaScript also supports functions without any special declarative requirements. Functions can be properties of objects, executing as loosely typed methods.

JavaScript is a very free-form language compared to Java. You do not have to declare all variables, classes, and methods. You do not have to be concerned with whether methods are public, private, or protected, and you do not have to implement interfaces. Variables, parameters, and function return types are not explicitly typed.

Hello world

To get started with writing JavaScript, open the Scratchpad and write your first "Hello world" JavaScript code:

function greetMe(yourName) 
  { alert("Hello " + yourName); } 
    greetMe("World");

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Variables

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Declaring Variables

You can declare a variable in three ways:
With the keyword var. For example, 

Var x = 42.

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This syntax can be used to declare both local and global variables.

By simply assigning it a value. For example, 
x = 42.

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This always declares a global variable. It generates a strict JavaScript warning. You shouldn't use this variant.

With the keyword let. For example, 

let y = 13.
Variable scope

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if (true) { var x = 5; } console.log(x); //5

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Primitive data type