func-package

├── esm
│   ├── README.md
│   ├── custom-loader.mjs
│   ├── index.js
│   ├── lib
│   │   ├── data.json
│   │   ├── path.js
│   │   └── render.js
│   ├── package.json
│   └── view
│       ├── index.html
│       ├── index.html
│       └── todo.html

import Koa from 'koa';
import { render } from './lib/render.js';
import data from './lib/data.json';

let app = new Koa();
app.use((ctx, next) => {
    let view = ctx.url.substr(1);
    let content;
    if ( view === '' ) {
        content = render('index');
    } else if ( view === 'data' ) {
        content = data;
    } else {
        content = render(view);
    }
    ctx.body = content;
})
app.listen(3000, ()=>{
    console.log('the modules test server is starting');
})

node --experimental-modules  --loader ./custom-loader.mjs ./index.js

import url from 'url';
import path from 'path';
import process from 'process';
import fs from 'fs';

// 从package.json中
// 的dependencies、devDependencies获取项目所需npm模块信息
const ROOT_PATH = process.cwd();
const PKG_JSON_PATH = path.join( ROOT_PATH, 'package.json' );
const PKG_JSON_STR = fs.readFileSync(PKG_JSON_PATH, 'binary');
const PKG_JSON = JSON.parse(PKG_JSON_STR);
// 项目所需npm模块信息
const allDependencies = {
  ...PKG_JSON.dependencies || {},
  ...PKG_JSON.devDependencies || {}
}

//Node原生模信息
const builtins = new Set(
  Object.keys(process.binding('natives')).filter((str) =>
    /^(?!(?:internal|node|v8)\/)/.test(str))
);

// 文件引用兼容后缀名
const JS_EXTENSIONS = new Set(['.js', '.mjs']);
const JSON_EXTENSIONS = new Set(['.json']);

export function resolve(specifier, parentModuleURL, defaultResolve) {
  // 判断是否为Node原生模块
  if (builtins.has(specifier)) {
    return {
      url: specifier,
      format: 'builtin'
    };
  }

  // 判断是否为npm模块
  if ( allDependencies && typeof allDependencies[specifier] === 'string' ) {
    return defaultResolve(specifier, parentModuleURL);
  }

  // 如果是文件引用，判断是否路径格式正确
  if (/^\.{0,2}[/]/.test(specifier) !== true && !specifier.startsWith('file:')) { 
    throw new Error(
      `imports must begin with '/', './', or '../'; '${specifier}' does not`);
  }

  // 判断是否为*.js、*.mjs、*.json文件
  const resolved = new url.URL(specifier, parentModuleURL);
  const ext = path.extname(resolved.pathname);
  if (!JS_EXTENSIONS.has(ext) && !JSON_EXTENSIONS.has(ext)) {
    throw new Error(
      `Cannot load file with non-JavaScript file extension ${ext}.`);
  }

  // 如果是*.js、*.mjs文件
  if (JS_EXTENSIONS.has(ext)) {
    return {
      url: resolved.href,
      format: 'esm'
    };
  }
  
  // 如果是*.json文件
  if (JSON_EXTENSIONS.has(ext)) {
    return {
      url: resolved.href,
      format: 'json'
    };
  }

}

return {
  url: '',
  format: ''
}