Search code examples
node.jsimportmoduleexportprogramming-languages

Languages with a NodeJS/CommonJS style module system

I really like the way NodeJS (and it's browser-side counterparts) handle modules:

var $ = require('jquery');

var config = require('./config.json');

module.exports = function(){};

module.exports = {...}

I am actually rather disappointed by the ES2015 'import' spec which is very similar to the majority of languages.

Out of curiosity, I decided to look for other languages which implement or even support a similar export/import style, but to no avail.

Perhaps I'm missing something, or more likely, my Google Foo isn't up to scratch, but it would be really interesting to see which other languages work in a similar way.

Has anyone come across similar systems? Or maybe someone can even provide reasons that it isn't used all that often.

Solution

  • It is nearly impossible to properly compare these features. One can only compare their implementation in specific languages. I collected my experience mostly with the language Java and nodejs.

    I observed these differences:

    • You can use require for more than just making other modules available to your module. For example, you can use it to parse a JSON file.
    • You can use require everywhere in your code, while import is only available at the top of a file.
    • require actually executes the required module (if it was not yet executed), while import has a more declarative nature. This might not be true for all languages, but it is a tendency.
    • require can load private dependencies from sub directories, while import often uses one global namespace for all the code. Again, this is also not true in general, but merely a tendency.

    Responsibilities

    As you can see, the require method has multiple responsibilities: declaring module dependencies and reading data. This is better separated with the import approach, since import is supposed to only handle module dependencies. I guess, what you like about being able to use the require method for reading JSON is, that it provides a really easy interface to the programmer. I agree that it is nice to have this kind of easy JSON reading interface, however there is no need to mix it with the module dependency mechanism. There can just be another method, for example readJson(). This would separate the concerns, so the require method would only be needed for declaring module dependencies.

    Location in the Code

    Now, that we only use require for module dependencies, it is a bad practice to use it anywhere else than at the top of your module. It just makes it hard to see the module dependencies when you use it everywhere in your code. This is why you can use the import statement only on top of your code.

    I don't see the point where import creates a global variable. It merely creates a consistent identifier for each dependency, which is limited to the current file. As I said above, I recommend doing the same with the require method by using it only at the top of the file. It really helps to increase the readability of the code.

    How it works

    Executing code when loading a module can also be a problem, especially in big programs. You might run into a loop where one module transitively requires itself. This can be really hard to resolve. To my knowledge, nodejs handles this situation like so: When A requires B and B requires A and you start by requiring A, then:

    • the module system remembers that it currently loads A
    • it executes the code in A
    • it remembers that is currently loads B
    • it executes the code in B
    • it tries to load A, but A is already loading
    • A is not yet finished loading
    • it returns the half loaded A to B
    • B does not expect A to be half loaded

    This might be a problem. Now, one can argue that cyclic dependencies should really be avoided and I agree with this. However, cyclic dependencies should only be avoided between separate components of a program. Classes in a component often have cyclic dependencies. Now, the module system can be used for both abstraction layers: Classes and Components. This might be an issue.

    Next, the require approach often leads to singleton modules, which cannot be used multiple times in the same program, because they store global state. However, this is not really the fault of the system but the programmers fault how uses the system in the wrong way. Still, my observation is that the require approach misleads especially new programmers to do this.

    Dependency Management

    The dependency management that underlays the different approaches is indeed an interesting point. For example Java still misses a proper module system in the current version. Again, it is announced for the next version, but who knows whether this will ever become true. Currently, you can only get modules using OSGi, which is far from easy to use.

    The dependency management underlaying nodejs is very powerful. However, it is also not perfect. For example non-private dependencies, which are dependencies that are exposed via the modules API, are always a problem. However, this is a common problem for dependency management so it is not limited to nodejs.

    Conclusion

    I guess both are not that bad, since each is used successfully. However, in my opinion, import has some objective advantages over require, like the separation of responsibilities. It follows that import can be restricted to the top of the code, which means there is only one place to search for module dependencies. Also, import might be a better fit for compiled languages, since these do not need to execute code to load code.