More customizable module resolution

[tblaisot]

Posted on nodejs/node#4584

I am coming from java and maven world and after using npm and node in multi-module project, i think a lot of improvement could be done with module resolution allowing better work with local modules:
Could node have a parameter (let suppose --module_resolution is this command line option ) to be able to provided a file with mapping between module name and path into the filesystem (like the NODE_PATH variable) ?

This file could be a map like

{
    "moduleA" : {
        "path" : "/path/to/moduleA/version/1.0.0",
        "resolve" :  {
            "moduleA" : {
                "path" : "/path/to/moduleB/version/2.0.0",
                "resolve" : { ... an so on ... }
            }
        }
    },
    "moduleB" : {
        "path" : "/path/to/moduleB/version/1.0.0",
        "resolve" : { ... an so on ... }
    }

}

//This map will look a lot like npm-shrinkwrap.json file but with local path instead of url
//I will suppose this file is named module-resolution.json

so running

node /path/to/moduleMain/index.js --module_resolution=/path/to/moduleMain/module-resolution.json

will have this effect:

• in the launched module (moduleMain/index.js):
  • require("moduleA") will resolve to "/path/to/moduleA/version/1.0.0"
  • require("moduleB") will resolve to "/path/to/moduleB/version/1.0.0"
• but in moduleA :
  • require("moduleB") will resolve to "/path/to/moduleB/version/2.0.0"

Avantages:

• allow the same flexibility than nested node_modules folders (multiple version of the same module name, each module being able to import the choosen version)
• optimize module resolution because there no more need to iterate going up the filesystem tree to find a node-module folder containing the module
• all modules and version can be installed in one local repository (like npm-cache but with exploded tgz)
• no need for linking global modules into local node_module folder
• no need to fetch and install the same dependencies multiple time for multiple modules with the same dependency
• no more need for linking folders for local dev with multiple modules
• no need for npm-dedupe

This file could be generated by modules managers like npm or bower.
Differents modules managers can handle multiple versions differently easily (if a module manager want to only allow one version of a module to be used, it just have to modify this map)

This file could be agnostic of the resolution of the js file from the module name so externals tools could use this file for require resolution:
algorithme like
looking for main field in /resolved/path/from/the/map/package.json for node or webpack
or
looking for typing field in /resolved/path/from/the/map/package.json for typescript compiler
can be let to the executor discretion.

[hediet]

I'd suggest to even allow dependency graphs rather than just dependency trees, so that module instances can be shared.
I hacked together a proof of concept that hijacks nodejs' module resolution algorithm: https://github.com/hediet/HowNodeJsModulesShouldHaveBeenDesigned