Node.js の簡単な分析で、require 関数にフックを追加する方法を詳細に学習します。

// lib/internal/modules/cjs/loader.js
Module._extensions['.node'] = function(module, filename) {
  return process.dlopen(module, path.toNamespacedPath(filename));
};

// lib/internal/modules/cjs/loader.js
Module._extensions['.json'] = function(module, filename) {
 const content = fs.readFileSync(filename, 'utf8');
 try {
    module.exports = JSONParse(stripBOM(content));
 } catch (err) {
   err.message = filename + ': ' + err.message;
   throw err;
 }
};

// lib/internal/modules/cjs/loader.js
Module._extensions['.js'] = function(module, filename) {
  // If already analyzed the source, then it will be cached.
  const cached = cjsParseCache.get(module);
  let content;
  if (cached?.source) {
    content = cached.source;
    cached.source = undefined;
  } else {
    content = fs.readFileSync(filename, 'utf8');
  }
  // 省略部分代码
  module._compile(content, filename);
};

// register.js
const fs = require("fs");
const Module = require("module");
const { transformSync } = require("esbuild");

Module._extensions[".ts"] = function (module, filename) {
  const content = fs.readFileSync(filename, "utf8");
  const { code } = transformSync(content, {
    sourcefile: filename,
    sourcemap: "both",
    loader: "ts",
    format: "cjs",
  });
  module._compile(code, filename);
};

// index.ts
const add = (a: number, b: number) => a + b;

console.log("add(a, b) = ", add(3, 5));

$ node -r ./register.js index.ts

add(a, b) =  8

  module.exports = "hello world";

npm install --save pirates

// register.js
const addHook = require("pirates").addHook;

const revert = addHook(
  (code, filename) => code.replace("@@foo", "console.log('foo');"),
  { exts: [".js"] }
);

// index.js
console.log("@@foo")

$ node -r ./register.js index.js

console.log('foo');

// src/index.js
export function addHook(hook, opts = {}) {
  let reverted = false;
  const loaders = []; // 存放新的loader
  const oldLoaders = []; // 存放旧的loader
  let exts;

  const originalJSLoader = Module._extensions['.js']; // 原始的JS Loader 

  const matcher = opts.matcher || null;
  const ignoreNodeModules = opts.ignoreNodeModules !== false;
  exts = opts.extensions || opts.exts || opts.extension || opts.ext 
    || ['.js'];
  if (!Array.isArray(exts)) {
    exts = [exts];
  }
  exts.forEach((ext) { 
    // ... 
  }
}

exts.forEach((ext) => {
    if (typeof ext !== 'string') {
      throw new TypeError(`Invalid Extension: ${ext}`);
    }
    // 获取已注册的loader，若未找到，则默认使用JS Loader
    const oldLoader = Module._extensions[ext] || originalJSLoader;
    oldLoaders[ext] = Module._extensions[ext];

    loaders[ext] = Module._extensions[ext] = function newLoader(
	  mod, filename) {
      let compile;
      if (!reverted) {
        if (shouldCompile(filename, exts, matcher, ignoreNodeModules)) {
          compile = mod._compile;
          mod._compile = function _compile(code) {
			// 这里需要恢复成原来的_compile函数，否则会出现死循环
            mod._compile = compile;
			// 在编译前先执行用户自定义的hook函数
            const newCode = hook(code, filename);
            if (typeof newCode !== 'string') {
              throw new Error(HOOK_RETURNED_NOTHING_ERROR_MESSAGE);
            }

            return mod._compile(newCode, filename);
          };
        }
      }

      oldLoader(mod, filename);
    };
});