Importance of JavaScript

All the client-side frameworks are based on JavaScript. Being an ASP.NET developer, we should have solid concepts of JavaScript before using or integrating them in our applications. JavaScript is the client-side scripting language and one of the most popular programming languages of all times that run on top of a browser. When working on a web development project, this language serves you in many better ways to make user interface (UI) responsive. With JavaScript, you can manipulate HTML page Document Object Model (DOM) elements, call server-side code through ajaxified requests and bring new rich experience to your customers. There are many innovations being done at the core JavaScript library, and different frameworks and various libraries have been developed.

What is JavaScript?

JavaScript is a programming language created in 1995 by Brenden Eich. Initially, it was only supported by Netscape Browser, but later they decided to release a standard known as ECMA specification to let other browsers implement and provide engines to execute JavaScript on their browsers. The reason for providing the standard is to have the complete specification details for the party to follow and provide consistent behavior.

Earlier it was only targeted to execute on browsers and perform client-side operations that work with HTML pages and provide features such as manipulating DOM elements and defining event handlers and other functionalities. Later, and in recent years, it has become a powerful language and not only bounded to the client-side operations. With Node.js, we can use JavaScript on server side and there are various modules and plugins provided by Node to perform I/O operations, server-side events, and more.

Comparing runtimes

As this book is targeted for .NET developers, let's compare the JavaScript runtime with .NET runtime. There are a few things in common, but the basic runtime is different.

In .NET, Common Language Runtime (CLR) does the just-in-time (JIT) compilation on the code that is running and provides memory management. JIT compilation is done on the Intermediate Language (IL) code that is generated once you build your project.

In the JavaScript world, browser engine is the runtime for the JavaScript language. Every browser interprets JavaScript in its own way, but follows the ECMA scripting standards. Different browsers have different implementations, for example, Microsoft Edge uses Chakra engine, Chrome uses V8, and Firefox has Monkey engines. Initially, JavaScript was implemented as an interpreted language, but few modern browsers now perform JIT compilation. Every engine provides a set of services such as memory management, compilation, and processing.

The following diagram shows the comparison between both the architectures:

The JavaScript parser parses and tokenizes the JavaScript code into a syntax tree. All the browsers, except Google V8, parse the syntax tree and generate a bytecode that finally converts into a machine code through JIT compilation. On the other hand, Google V8 engine parses the syntax tree and instead of generating a bytecode first, it directly generates the machine code.

The .NET source code is compiled by its own language compiler, such as C# or VB.NET compiler and passes through the several stages of the compiler pipeline to generate an IL code. This IL code is then read by the JIT compiler that generates the native machine code.