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.

Java is a class-based programming language designed for fast execution and type safety. Type safety means, for instance, that you can't cast a Java integer into an object reference or access private memory by corrupting Java bytecodes. Java's class-based model means that programs consist exclusively of classes and their methods. Java's class inheritance and strong typing generally require tightly coupled object hierarchies. These requirements make Java programming more complex than JavaScript programming.

In contrast, JavaScript descends in spirit from a line of smaller, dynamically typed languages such as HyperTalk and dBASE. These scripting languages offer programming tools to a much wider audience because of their easier syntax, specialized built-in functionality, and minimal requirements for object creation.

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

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

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

You can declare a variable in two ways:

• With the keyword var . For example, var x = 42. This syntax can be used to declare both local and global variables, depending on the execution context.
• With the keyword const or let . For example, let y = 13 . This syntax can be used to declare a block-scope local variable.

When you declare a variable outside of any function, it is called a global variable, because it is available to any other code in the current document. When you declare a variable within a function, it is called a local variable, because it is available only within that function.

JavaScript before ECMAScript 2015 does not have block statement scope; rather, a variable declared within a block is local to the function (or global scope) that the block resides within. For example the following code will log 5, because the scope of x is the global context (or the function if the following codes are part of the function), not the immediate if statement block.

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

This behavior changes, when using the let declaration introduced in ECMAScript 2015.

if (true) {
  let y = 5;
}
console.log(y); // ReferenceError: y is not defined

Global variables are in fact properties of the global object. In web pages, the global object is window, so you can set and access global variables using the window.variable syntax.

Consequently, you can access global variables declared in one window or frame from another window or frame by specifying the window or frame name. For example, if a variable called phoneNumber is declared in a document, you can refer to this variable from an iframe as parent.phoneNumber.

You can create a read-only, named constant with the const keyword. The syntax of a constant identifier is the same as for a variable identifier: it must start with a letter, underscore or dollar sign ($) and can contain alphabetic, numeric, or underscore characters.

const PI = 3.14;

A constant cannot change value through assignment or be re-declared while the script is running. It must be initialized to a value.

The scope rules for constants are the same as those for let block-scope variables. If the const keyword is omitted, the identifier is assumed to represent a variable.

The latest ECMAScript standard defines eight data types:

• Seven data types that are primitives:
  • Boolean. true and false.
  • null. A special keyword denoting a null value. Because JavaScript is case-sensitive, null is not the same as Null , NULL, or any other variant.
  • undefined. A top-level property whose value is not defined.
  • Number. An integer or floating point number. For example: 42 or 3.14159.
  • BigInt. An integer with arbitrary precision. For example:9007199254740992n .
  • String. A sequence of characters that represent a text value. For example: "Howdy"
  • Symbol. (new in ECMAScript 2015) A data type whose instances are unique and immutable.
• and Object

Use the if statement to execute a statement if a logical condition is true. Use the optional else clause to execute a statement if the condition is false. An if statement looks as follows:

if (condition) {
  statement_1;
}
else {
  statement_2;
}

A switch statement allows a program to evaluate an expression and attempt to match the expression's value to a case label. If a match is found, the program executes the associated statement. A switch statement looks as follows:

switch (expression) {
  case label_1:
   statements_1
   [break;]
  case label_2:
   statements_2
   [break;]
  ...
default:
   statements_def
   [break;]
}

A while statement executes its statements as long as a specified condition evaluates to true. A while statement looks as follows:

while (condition)
  statement

If the condition becomes false, statement within the loop stops executing and control passes to the statement following the loop.

The condition test occurs before statement in the loop is executed. If the condition returns true, statement is executed and the condition is tested again. If the condition returns false, execution stops and control is passed to the statement following while .

To execute multiple statements, use a block statement ({ ... }) to group those statements.

A function definition (also called a function declaration, or function statement) consists of the function keyword, followed by:

• The name of the function.
• A list of parameters to the function, enclosed in parentheses and separated by commas.
• The JavaScript statements that define the function, enclosed in curly brackets, { }.

For example, the following code defines a simple function named square:

function square(number) {
  return number * number;
}

All the Documentation in this page is taken from MDN web docs.