WORKING WITH PRE-ES6 MODULE LOADERS
Prior to the introduction of native ES6 module support, JavaScript codebases using modules essentially wanted to use a language feature that was not available by default. Therefore, codebases would be written in a module syntax that conformed to a certain specification, and separate module tooling would serve to bridge the gap between the module syntax and the JavaScript runtime. The module syntax and bridging took a number of different forms, usually either a supplementary library in the browser or preprocessing at build time.
CommonJS
The CommonJS specification outlines a convention for module definition that uses synchronous declarative dependencies. This specification is primarily intended for module organization on the server, but it can also be used to define dependencies for modules that will be used in the browser. CommonJS module syntax will not work natively in the browser.
