JavaScript(part-01)

 What is JavaScript??


JavaScript is a lightweight, interpreted programming language. It is designed for creating network-centric applications. It is complimentary to and integrated with Java. JavaScript is very easy to implement because it is integrated with HTML. It is open and cross-platform.

JavaScript was first known as LiveScript, but Netscape changed its name to JavaScript, possibly because of the excitement being generated by Java. JavaScript made its first appearance in Netscape 2.0 in 1995 with the name LiveScript. The general-purpose core of the language has been embedded in Netscape, Internet Explorer, and other web browsers.


Why to Learn JavaScript ?


  • JavaScript is the most popular programming language in the world and that makes it a programmer’s great choice. Once you learnt JavaScript, it helps you developing great front-end as well as back-end software using different JavaScript based frameworks like jQuery, Node.JS etc.
  • JavaScript is everywhere, it comes installed on every modern web browser and so to learn JavaScript you really do not need any special environment setup. For example Chrome, Mozilla Firefox , Safari and every browser you know as of today, supports JavaScript.
  • JavaScript helps you create really beautiful and crazy fast websites. You can develop your website with a console like look and feel and give your users the best Graphical User Experience.
  • JavaScript usage has now extended to mobile app development, desktop app development, and game development. This opens many opportunities for you as JavaScript Programmer.


JavaScript frameworks


There are many useful Javascript frameworks and libraries available:

  • Angular
  • React
  • jQuery
  • Vue.js
  • Ext.js
  • Ember.js
  • Meteor
  • Mithril
  • Node.js
  • Polymer
  • Aurelia
  • Backbone.js


Applications of JavaScript Programming


  • Client side validation - This is really important to verify any user input before submitting it to the server and Javascript plays an important role in validting those inputs at front-end itself.
  • Manipulating HTML Pages - Javascript helps in manipulating HTML page on the fly. This helps in adding and deleting any HTML tag very easily using javascript and modify your HTML to change its look and feel based on different devices and requirements.
  • User Notifications - You can use Javascript to raise dynamic pop-ups on the webpages to give different types of notifications to your website visitors.
  • Back-end Data Loading - Javascript provides Ajax library which helps in loading back-end data while you are doing some other processing. This really gives an amazing experience to your website visitors.
  • Presentations - JavaScript also provides the facility of creating presentations which gives website look and feel. JavaScript provides RevealJS and BespokeJS libraries to build a web-based slide presentations.
  • Server Applications - Node JS is built on Chrome's Javascript runtime for building fast and scalable network applications. This is an event based library which helps in developing very sophisticated server applications including Web Servers.

Advantages of JavaScript


  • Less server interaction − You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.
  • Immediate feedback to the visitors − They don't have to wait for a page reload to see if they have forgotten to enter something.
  • Increased interactivity − You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.
  • Richer interfaces − You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.

Limitations of JavaScript


We cannot treat JavaScript as a full-fledged programming language. It lacks the following important features −

  • Client-side JavaScript does not allow the reading or writing of files. This has been kept for security reason.
  • JavaScript cannot be used for networking applications because there is no such support available.
  • JavaScript doesn't have any multi-threading or multiprocessor capabilities.


Case Sensitivity


JavaScript is a case-sensitive language. This means that the language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.

JavaScript Data types


One of the most fundamental characteristics of a programming language is the set of data types it supports. These are the type of values that can be represented and manipulated in a programming language.

JavaScript allows you to work with three primitive data types −

  • Numbers, eg. 123, 120.50 etc.
  • Strings of text e.g. "This text string" etc.
  • Boolean e.g. true or false.

JavaScript also defines two trivial data types, null and undefined, each of which defines only a single value. In addition to these primitive data types, JavaScript supports a composite data type known as object. We will cover objects in detail in a separate chapter.

JavaScript Variables Scopes


The scope of a variable is the region of your program in which it is defined. JavaScript variables have only two scopes.

  • Global Variables − A global variable has global scope which means it can be defined anywhere in your JavaScript code.
  • Local Variables − A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.

Within the body of a function, a local variable takes precedence over a global variable with the same name. If you declare a local variable or function parameter with the same name as a global variable, you effectively hide the global variable.


JavaScript Variable Names

  • You should not use any of the JavaScript reserved keywords as a variable name. These keywords are mentioned in the next section. For example, break or boolean variable names are not valid.
  • JavaScript variable names should not start with a numeral (0-9). They must begin with a letter or an underscore character. For example, 123test is an invalid variable name but _123test is a valid one.
  • JavaScript variable names are case-sensitive. For example, Name and name are two different variables.


What is an Operator?


Let us take a simple expression 4 + 5 is equal to 9. Here 4 and 5 are called operands and ‘+’ is called the operator. JavaScript supports the following types of operators.

  • Arithmetic Operators
  • Comparison Operators
  • Logical (or Relational) Operators
  • Assignment Operators
  • Conditional (or ternary) Operators

If – else

if statement

The if statement is the fundamental control statement that allows JavaScript to make decisions and execute statements conditionally.

Syntax

The syntax for a basic if statement is as follows −

if (expression) {
   Statement(s) to be executed if expression is true
}

if...else statement

The 'if...else' statement is the next form of control statement that allows JavaScript to execute statements in a more controlled way.

Syntax

if (expression) {
   Statement(s) to be executed if expression is true
} else {
   Statement(s) to be executed if expression is false
}

if...else if... statement

The if...else if... statement is an advanced form of if…else that allows JavaScript to make a correct decision out of several conditions.

Syntax

The syntax of an if-else-if statement is as follows −

if (expression 1) {
   Statement(s) to be executed if expression 1 is true
} else if (expression 2) {
   Statement(s) to be executed if expression 2 is true
} else if (expression 3) {
   Statement(s) to be executed if expression 3 is true
} else {
   Statement(s) to be executed if no expression is true
}

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