Modular Programming in JavaScript

The principles and evolution of modular programming in JavaScript. The primary goal of modular programming is to facilitate large-scale program development by integrating various code modules from different sources. It focuses on hiding the implementation details and keeping the global namespace clean to prevent accidental alteration of variables, functions, and classes defined by other modules.

Historically, JavaScript didn’t offer built-in support for modules. Developers utilized objects, classes, and closures to achieve a degree of modularity in large code bases. This limited modularity was supplemented by code-bundling tools, eventually leading to a practical form of modularity via a require() function. This approach was adopted by Node.js and became fundamental to its programming environment. However, it was never officially integrated into the JavaScript language.

Instead, with the advent of ES6 (ECMAScript 2015), JavaScript introduced import and export keywords to support modules natively. Although they have been a part of the language specification for years, their actual implementation in web browsers and Node.js was comparatively recent. As of the current state, the practical implementation of JavaScript modularity heavily depends on code-bundling tools.

So, to summarize, JavaScript’s modular programming has evolved over time from leveraging closures and objects to implementing an official module system with the import and export keywords in ES6. Despite this, code bundling tools continue to play a significant role in managing modularity in practical JavaScript development.

The text discusses the introduction of import and export keywords in JavaScript with ES6 (ECMAScript 2015), marking the support for true modularity as a core language feature. It equates the concept of ES6 modularity with Node.js modularity, where each file serves as a separate module. Elements like constants, variables, functions, and classes declared within a file are private to that module unless explicitly exported. These exported values from one module can then be used in other modules that import them.

However, ES6 modules differ from Node.js modules in their export and import syntax and how they are defined in web browsers. Also, ES6 modules aren’t identical to conventional JavaScript scripts. Conventional scripts place top-level declarations of variables, functions, and classes in a shared global context. In contrast, each ES6 module has its private context and can use import and export statements to control accessibility.

Moreover, ES6 modules enforce ‘strict mode’ by default. This means developers no longer need to specify “use strict” in their code when using ES6 modules. It also implies certain restrictions, like the inability to use the with statement, the arguments object, or undeclared variables. ES6 modules have stricter rules than the conventional strict mode. For instance, in strict mode, the value of this is undefined when a function is invoked as a function. However, in ES6 modules, this is undefined even in top-level code, unlike scripts in web browsers and Node.js, which set this to the global object.