Revised module resolution

[vladima]

This PR consolidates all module resolution logic in one place so there is only one entry point for everything that needs to resolve module names. Currently it is function getDefaultModuleNameResolver function that accepts CompilerOptions and returns back an instance of ModuleResolved which is the alias for export type ModuleNameResolver = (moduleName: string, containingFile: string, options: CompilerOptions, host: ModuleResolutionHost) => ResolvedModule;. Default module resolver is used in several places:

• preProcessFile that is called from VS to determine dependencies of the file
• createProgram - at this point default module resolver will be requested if host wants to process with default module name resolution rules. NOTE: host can override built-in rules by implementing resolveModuleNames?(moduleNames: string[], containingFile: string): string[]; function that accepts a list of imported names in the file and returns a list of resolved file names in the same order.
• resolveModuleNames implementation in LSHost for 'tsserver'.

Now CompilerHost / LanguageServiceHost can implement resolveModuleNames and be completely in control of how module names are resolved. Alternatively they might want to use built-in rules - in this case they should implement getModuleResolutionHost, return value of this function will be used by default module resolver to ask file system related questions.

[vladima]

In theory existing implementation of LanguageServiceHost interface should just work because of this. Pinging @basarat so he can be aware of this change.

[basarat]

Excited 🌹 !

[vladima]

adding @dbaeumer, @derekcicerone, @mhegazy

[mhegazy]

i would use Harness.IO.fileExists

[vladima]

done

[derekcicerone-zz]

Neat, looks like this could be used for modules which need to be resolved across project boundaries perhaps.

[mhegazy]

 function resolveModuleName(moduleName: string, containingFile: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost): ResolvedModule {
    switch(compilerOptions.module) {
         case ModuleKind.CommonJS:
               resolveCommonJSModuleName(....);
             //...
    }
}   

[vladima]

done

[mhegazy]

👍

[jbrantly]

Just wanted to comment that I recently added support for this to ts-loader for resolving modules the same way that webpack does (setting up aliases and such) and it's fantastic. This is a great addition to TypeScript! Good work.

[vladima]

Great to hear that!