JavaScript Introduction
JavaScript is a cross-platform, object-oriented scripting language used to make webpages interactive (e.g., having complex animations, clickable buttons, popup menus, etc.). There are also more advanced server side versions of JavaScript such as Node.
JavaScript is a very powerful client-side scripting language. JavaScript is used mainly for enhancing the interaction of a user with the webpage. In other words, you can make your webpage more lively and interactive, with the help of JavaScript. JavaScript is also being used widely in game development and mobile application development.
JavaScript History
JavaScript was developed by Brendan Eich in 1995, which appeared in Netscape, a popular browser of that time.
The language was initially called LiveScript and was later renamed JavaScript. Some programmers think JavaScript and Java are the same. In fact, JavaScript and Java are very much unrelated. Java is a very complex programming language whereas JavaScript is only a scripting language. The syntax of JavaScript is mostly influenced by the programming language C.
How to Run JavaScript?
JavaScript as a scripting language cannot run on its own. When a user requests an HTML page with JavaScript in it, the script is sent to the browser and it is up to the browser to execute it. The main advantage of JavaScript is that all modern web browsers support JavaScript. Also, JavaScript runs on any operating system including Windows, Linux or Mac.
Tools You Need
To start with, you need a text editor to write your code and a browser to display the web pages you develop. You can use a text editor of your choice including Notepad++, Visual Studio Code, Sublime Text, Atom or any other text editor you are comfortable with. You can use any web browser including Google Chrome, Firefox, Microsoft Edge, Internet Explorer etc.
A Simple JavaScript Program
You should place all your JavaScript code within <script> tags (<script> and </script>) if you are keeping your JavaScript code within the HTML document itself. This helps your browser distinguish your JavaScript code from the rest of the code. it is highly recommended that you specify the scripting language you use. You have to use the type attribute within the <script> tag and set its value to text/javascript like this:
<script type=”text/javascript”>
JavaScript Features and Attributes
General language features
Scripting Language
JavaScript is a lightweight scripting language made for client-side execution on the browser. Since it is not designed as a general-purpose language and is specially engineered for web applications, the set of libraries is also geared primarily towards web applications.
Interpreter Based
JavaScript is an interpreted language instead of a compiled one. In that sense, it is closer to languages like Ruby and Python. The browser interprets JavaScript’s source code, line by line and runs it. In contrast, a compiled language needs to be compiled into a byte-code code executable. Java and C++ are examples of compiled languages.
Event Handling
An event is an action or an occurrence in a system that communicates about said occurrence so that you can respond to it somehow. For example, a user clicks on a button, and the system tells you to respond to the button click event with an action, say an information box.
JavaScript enables you to handle events and even generate custom events.
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Light Weight
JavaScript isn’t a compiled language, so it doesn’t get converted to byte-code beforehand. However, it does follow a paradigm called Just-In-Time (JIT) Compilation. Meaning it gets converted to bytecode just as it’s about to run. This enables JS to be lightweight. Even less powerful devices are capable of running JavaScript.
Case Sensitive
JavaScript is highly case sensitive. All keywords, variables, functions names and other identifiers can and must only follow a consistent capitalisation of letters. E.g.:
var hitCounter = 5
var hitcounter = 5
Here variables hitCounter and hitcounter are both different variables because of the difference in the case. Also, all keywords such as “var” are case sensitive.
Control Statements
JavaScript is equipped with control statements like if-else-if, switch-case, and loops like for, while, and do-while loops. These control statements make it a powerful programming language, enabling its user to write complex logic.
Objects as first-class citizens
All non-primitive data types in JavaScript are actually objects, i.e. data types like Arrays, Functions, Symbols etc. inherit all the properties of the Object prototype.
The term first-class citizen means “being able to do what everyone else can do”. In JavaScript Objects prototype is the base prototype of all. They can be passed as reference, returned in a function, and assigned to variables for manipulation. This concept is also extended to functions as Object is also the prototype of functions.
Functions as First-class citizens(supports functional programming)
What do we mean by functions as first-class objects/citizens? Functions that return a function are called Higher Order Functions, which JavaScript supports. Functions as first-class citizens simply mean functions enjoy similar behaviour from the JavaScript interpreter as that of objects or any other variable. Meaning, we can pass them into arguments (pass by reference), return them by another function, and assign them to a variable as a value.
Dynamic Typing
JavaScript is a dynamically typed language. It means that the JS interpreter does not require explicit declaration of the variables before they are used. E.g.:
var dynamicType = “a string”
dynamicType = 5
Here we can see the same variable dynamicType can contain either a string or an integer with the same variable declaration. That is, the variable type does not need to be declared during creation or assignment.
Language Use Features
Client-side validations
JavaScript is heavily used for easing the interactions between users and web applications. Towards this, validation plays a significant role. Validations can guide users to fill forms correctly with valid data and prevent spam submissions.
Platform Independent
JavaScript is platform-independent; it can run on any computer irrespective of the operating systems used. JavaScript is built into Netscape 2.0 and greater. Since the JavaScript interpreter is part of Netscape, it runs on platforms that support Netscape, which means that a piece of Javascript code will give the same output with the same setup on all platforms.
Async Processing
JavaScript supports asynchronous behaviour through the use of Promises and Async functions. In Promises, we can make a request and attach a .then() clause to it, which only executes on the completion of the promise. The other alternative is to use the async-await syntax functions; async functions don’t execute sequentially but parallelly, which positively affects pages’ processing time and responsiveness.
Prototype-based
JavaScript is a prototype-based scripting language, which means it uses the prototypal inheritance model instead of the commonly known class inheritance. This means instead of creating classes and deriving objects from them, we define the Objects prototype and use this object prototype to create more Objects of the same type. This behaviour is similar to the Object factory method, and thus such a design pattern is commonly seen in JavaScript code.
