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Preprocessor in JavaScript

Preprocessor in JavaScript


Introduction

A preprocessor is a program or a set of codes that uses an arbitrary source file as its input data and gives output data. Another program as an input can then use this output data. Preprocessors convert files into a format that a browser or similar program is able to read or understand easily. The output delivered is a preprocessed form of input data. There are various kinds of preprocessors and the kind of processing they do depends on their characteristics/type. They can perform simple tasks such as running a macro as well as it performs highly complex tasks such as substituting a part of an entire programming language.

Preprocessors are usually of two types. They are either
 compilers or transpilers. Compilers are preprocessors that convert an entirely different language into JavaScript transpilers and convert a new language or a superset to standards. Transpilers resource-to-source compilers. They read source code written in one programming language and produce the equivalent code in another. Languages that transpile to JavaScript are compile-to-JS languages. ECMAScript 6 (ES6) and CoffeeScript are popular examples of transpilers and preprocessors are specifically designed for JavaScript. Lexical (language) preprocessors are flexible, which means, they are preprocessors with lexical analysis rules that can be customized and can be used with JavaScript. It is advisable to use preprocessors that have been designed for a language having a similar syntax as JavaScript. For example, C or C++ preprocessor. Let us now explore the Google Chrome development tool, also called Chrome DevTools, which has a JavaScript preprocessing feature.

What are Preprocessors

Preprocessors are software program equipment that remodels source code written in a single format into every other layout before it is completed. In the context of JavaScript, preprocessors are typically used to increase the language's syntax by introducing new capabilities or simplifying present ones. Preprocessors work by taking the source code written inside the preprocessor's particular syntax and converting it into valid JavaScript code that may be understood and executed through web browsers.

How do Preprocessors Work

Preprocessors usually comply with a three-step method to convert the preprocessor-specific code into JavaScript:

  • Parsing: The preprocessor parses the supply code written in its precise syntax and breaks it down into individual components which include tokens, expressions, and statements. This step involves reading the code's shape and know-how it's intended which means.

  • Transformation: Once the code is parsed, the preprocessor applies a sequence of alterations based totally on the predefined guidelines and configurations. These alterations alter the code to conform to JavaScript syntax and semantics.

  • Code Generation: After the differences are carried out, the preprocessor generates the very last JavaScript code. This code is equal to the authentic preprocessor-unique code but in a layout that can be understood and done by using internet browsers.

Benefits of Using Preprocessors

Preprocessors provide several advantages to JavaScript builders:

  • Enhanced Syntax: Preprocessors permit builders to apply superior syntax and language capabilities that might not be supported by all browsers. This allows developers to write cleaner and more efficient code without traumatic compatibility problems.

  • Improved Developer Experience: Preprocessors often offer extra equipment and utilities, which include linting, code formatting, and debugging guides. These functions enhance the developer's revel in and assist in trapping mistakes and troubles early within the development manner.

  • Code Organization and Reusability: Preprocessors provide functions like modules and namespaces, which help in organizing code into possible components. This promotes code reusability, maintainability, and scalability.

  • Language Extensibility: Preprocessors permit developers to increase JavaScript's abilities by introducing new language capabilities or simplifying existing ones. This ends in extra effective and efficient improvement workflows.

Chrome DevTools

Chrome DevTools is built with the Google Chrome browser and comprises a set of Web development tools that facilitate easier development. The Chrome DevTools JavaScript preprocessor captures JavaScript before compilation and allows JavaScript to be transcoded. Then, along with JavaScript that is injected into the page, the preprocess facilitates an artificial runtime to be constructed in JavaScript. This in combination with other functions present in Chrome DevTools extension API enables developers to create new JavaScript-related tools.

Chrome DevTools API

To use the script preprocessor, developers usually create code for a Chrome Devtools extension that will reload the Web page having the preprocessor installed. The JavaScript function, reload, takes three arguments, described as follows:

  • Source to preprocess: The Loader will bypass the cache for all inspected page resources loaded before the load event starts.

  • URL of the source: The string will override the value of the User-Agent HTTP  header, which is sent while loading the resources of the inspected page.

  • Script or function name of the event handler: The script will inject into every frame of the inspected page immediately upon load, before any of the frame's scripts.

Tip: Developers can visit the link http://developer.chrome.com/extensions/samples.html to view an example that illustrates the API call in a simple extension. Developers can download and extract the zip file. Then, they can use chrome://extensions in DeveloperMode and load the extracted extension. After this, when they open or reopenDevTools, they can see the Preprocessor panel having a reload button that triggers a simple preprocessor. Compilation of the preprocessor happens only once just before the first JavaScript is encountered. The preprocessor remains active until the page reloads or navigates. If the Web page is navigated back and then forward, it will not result in any preprocessing.


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