JavaScript is a cross-platform, object-oriented scripting language. It is a small and lightweight language.
Inside a host environment (for example, a web browser), JavaScript can be connected to the objects of its
environment to provide programmatic control over them.

JavaScript contains a standard library of objects, such as Array, Date, and Math, and a core set of language
elements such as operators, control structures, and statements. Core JavaScript can be extended for a variety
of purposes by supplementing it with additional objects; for example:

• Client-side JavaScript extends the core language by supplying objects to control a browser and its
  Document Object Model (DOM). For example, client-side extensions allow an application to place
  elements on an HTML form and respond to user events such as mouse clicks, form input, and page navigation.


• Server-side JavaScript extends the core language by supplying objects relevant to running JavaScript on a server.
  For example, server-side extensions allow an application to communicate with a database, provide continuity
  of information from one invocation to another of the application, or perform file manipulations on a server.

Client-side JavaScript extends the core language by supplying objects to control a browser and its Document Object Model (DOM).
For example, client-side extensions allow an application to place elements on an HTML form and respond to user events such as
mouse clicks, form input, and page navigation.

                    
                        HTML and CSS ARE framing languages 
                            and not programing languages.
                    
                

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

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

Select the code in the pad and hit Ctrl+R to watch it unfold in your browser!

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.

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

                    
                        var x = 42.
                    
                

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

By simply assigning it a value. For example,

                    
                        x = 42.
                    
                

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.
                    
                

This syntax can be used to declare a block scope local variable. See Variable scope below.

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 function (or global context) within which x is declared, not the block, which in this case is an if statement.

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

Six 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 undefined.
• Number. 42 or 3.14159.
• String. "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;
                                    }
                        
                    

condition can be any expression that evaluates to true or false. See Boolean for an explanation of what evaluates to true and false.
If condition evaluates to true, statement_1 is executed; otherwise, statement_2 is executed. statement_1 and statement_2 can be any statement,
including further nested if statements.
You may also compound the statements using else if to have multiple conditions tested in sequence, as follows:

                    
                    if (condition_1) {
                        } else if (condition_2) {
                            statement_2;
                        } else if (condition_n) {
                            statement_n;
                        } else {
                            statement_last;
                        }
                    
                

In the case of multiple conditions only the first logical condition which evaluates to true will be executed. To execute multiple statements, group
them within a block statement ({ ... }) . In general, it's good practice to always use block statements, especially when nesting if statements:

                    
                        if (condition) {
                            statement_1_runs_if_condition_is_true;
                            statement_2_runs_if_condition_is_true;
                            } else {
                                statement_3_runs_if_condition_is_false;
                                statement_4_runs_if_condition_is_false;
                                }
                    
                

It is advisable to not use simple assignments in a conditional expression, because the assignment can be confused with equality when glancing over the code.
For example, do not use the following code:

                    
                        if (x = y) {
                            /* statements here */
                            }
                    
                

If you need to use an assignment in a conditional expression, a common practice is to put additional parentheses around the assignment. For example:

                    
                        if ((x = y)) {
                            /* statements here */
                            }