It is a Universal Module Loader for JavaScript. If you've used RequireJs or a CommonJs bundler in the past, you have probably created modules.Configurable module loader enabling dynamic ES module workflows in browsers and NodeJS.
System.js is a tool in the Build Automation category of a tech stack.
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What are some alternatives to System.js?
npm is the command-line interface to the npm ecosystem. It is battle-tested, surprisingly flexible, and used by hundreds of thousands of JavaScript developers every day.
Visual Studio is a suite of component-based software development tools and other technologies for building powerful, high-performance applications.
Out of the box, IntelliJ IDEA provides a comprehensive feature set including tools and integrations with the most important modern technologies and frameworks for enterprise and web development with Java, Scala, Groovy and other languages.
A bundler for javascript and friends. Packs many modules into a few bundled assets. Code Splitting allows to load parts for the application on demand. Through "loaders" modules can be CommonJs, AMD, ES6 modules, CSS, Images, JSON, Coffeescript, LESS, ... and your custom stuff.