undo3d/support/node-esm-resolve-hook.mjs

Undo3D helps you build free-roaming 3D web apps where thousands of users can collaborate creatively in real time. Expect the first public beta mid-2019, and the first production release mid-2020.

//// Undo3D has a collection of browser shims which mimic the functionality of
//// various Node.js modules. This allows a single codebase to run on the server
//// and the client at the same time.
////
//// Historically, Node would load its filesystem utilities like this:
////   var fs = require('fs')
//// But browsers have no `require()`. Node can be made to understand this ES6:
////   import fs from 'fs'
//// But browsers need to import an actual path, like this:
////   import fs from '../deps/node_modules/undo3d-shim-browser/fs/all.mjs'
//// Browsers and Node happily import the above - in this case, a module which
//// simulates the filesystem using IndexDB. But we need Node to use its own
//// native 'fs' module instead. To make that happen, you’ll need to run Node
//// with the following option:
////   --loader ./support/node-esm-resolve-hook.mjs
////
//// Now Node imports its native 'fs' module, and browsers import the shim.
//// See https://nodejs.org/api/esm.html#esm_resolve_hook for more details.

//// Map the `pathEnd` of browser shims to Node native module names.
const shims = {
    'undo3d-shim-browser/assert/all.mjs': 'assert'
  // , 'undo3d-shim-browser/fs/all.mjs': 'fs' //@TODO
}

//// Set the default for the `parentModuleURL` argument.
const baseURL = new URL('file://')
baseURL.pathname = process.cwd() + '/'

//// Define the resolve hook.
export async function resolve(
    specifier
  , parentModuleURL = baseURL
  , defaultResolver
) {
    //// Get `pathEnd`, which is the last three path-parts of `specifier`:
    //// '../deps/node_modules/undo3d-shim-browser/fs/all.mjs'
    ////   -> ['..','deps','node_modules','undo3d-shim-browser','fs','all.mjs']
    ////   -> 'undo3d-shim-browser/fs/all.mjs'
    //// './just/a/random/module.mjs'
    ////   -> ['.','just','a','random','module.mjs']
    ////   -> 'a/random/module.mjs'
    const pathEnd = specifier.split('/').slice(-3).join('/')

    //// If `pathEnd` is recognised, use Node’s native module instead of the
    //// browser shim. For example, 'undo3d-shim-browser/fs/all.mjs' -> 'fs'.
    if (shims[pathEnd])
        return { format:'builtin', url:shims[pathEnd] }

    //// Otherwise, act as if we hadn’t hooked the resolver.
    return { format:'esm', url:new URL(specifier, parentModuleURL).href }
}