CommonJSの基礎 — require/module.exportsとexports

CJS：全体像

require() — 考え方

// CommonJS import uses require()
const cjsExample = [
  "const math = require(\\"./math.js\\");",
  "console.log(math.sum(2, 3));"
];

console.log("require() example:");
for (const line of cjsExample) console.log(line);

// Emulate a loaded module object
const loaded = { sum: function (a, b) { return a + b; } };
console.log("emulated require sum:", loaded.sum(2, 3));

module.exports

// module.exports sets what the module returns
const lines1 = [
  "function greet(name) { return \\"Hi \\" + name; }",
  "module.exports = greet;"
];

console.log("module.exports example:");
for (const l of lines1) console.log(l);

// Emulate consumption
function greet(name) { return "Hi " + name; }
const api1 = greet; // like module.exports = greet
console.log("use exported fn:", api1("Ayla"));

exportsのショートカット

// exports is a shortcut to module.exports (same object)
const lines2 = [
  "// Good: add properties",
  "exports.sum = function (a, b) { return a + b; };",
  "",
  "// Pitfall: reassigning exports breaks the link",
  "exports = function () {}; // WRONG"
];

console.log("exports shortcut:");
for (const l of lines2) console.log(l);

// Emulate good usage
const moduleObj = {};
const exportsRef = moduleObj;  // like exports === module.exports initially
exportsRef.sum = function (a, b) { return a + b; };
console.log("sum via exportsRef:", moduleObj.sum(2, 4));

複数のエクスポート

// Export an object with multiple members
const lines3 = [
  "function add(a, b) { return a + b; }",
  "function mul(a, b) { return a * b; }",
  "module.exports = { add, mul };"
];

console.log("multiple exports object:");
for (const l of lines3) console.log(l);

// Emulate consumption
const api2 = {
  add: function (a, b) { return a + b; },
  mul: function (a, b) { return a * b; }
};
console.log("use add/mul:", api2.add(1, 2), api2.mul(2, 3));

ESM ↔ CJSの概要

CJSエクスポート構文のクイズ

まとめ