Understanding SystemJS and import maps
Earlier in this chapter, we learned that ESM is arguably the best module system for JavaScript. After all, it is integrated into the JavaScript language. One of the reasons why other formats are still relevant today is backward compatibility.
Backward compatibility allows formats such as AMD or UMD to be used in older JavaScript runtimes, such as older versions of browsers such as Internet Explorer, but even if we don’t need backward compatibility, the alternative formats still have one or more advantages over ESM.
One of the core problems with ESM is that it does not define how modules are resolved. In fact, the only specified way to resolve a module is explicitly via the filesystem. When we used ESM, we explicitly stated our module imports, such as in ./b.js. As mentioned, we are not allowed to implicitly use something such as ./b or even just b.
When doing frontend development, the notion of dependencies has become quite elementary...
