node.js - javascript - Why is there a spec for sync and async modules?

This is a good question. It's a subject that caused a lot
of heated discussion in the Node community. To have a good understanding of what it's
all about you should read:

Now, answering your question -
Why is there a spec for sync and async modules? Because some usecases prefer the
synchronous loading of modules, like the server-side modules in Node.js where you want
to load everything you need before you start serving requests, and some usecases prefer
asynchronous loading of modules, like in the browser when you don't want to block the
rendering thread while you load the dependencies.

There is
really no option for synchronous loading in the browser because it would make the
browser not responsive.

You could argue that you might use
asynchronous loading on the server but then you'd either have to return promises instead
of modules by require() or it could take callbacks. Either way
it would make any complex code that uses a lot of modules much more
complicated.

Another issue is with the caching and mutation
of the already loaded modules. With synchronous module loading using
require you only load the
module once and any other
calls to require for the same module in the entire code base
(for that process) return a cached response, which is the same object every time. Any
part of the code can modify that object and it is available to any other part of the
code. Some usecases that use that feature would be much more complex to implement.
Additionally the order of loading and executing code would be harder to
predict.

To sum up the answer to your question, there are
arguments for both ways of loading modules and neither of those ways is a clear winner
for every scenario. Both are needed and both have some specs to standardize their
behavior.

Read the articles that I linked for more detailed
understanding.