行前準備

本篇文章，會大量用到 NodeJS HTTP Server 作為程式碼範例，為了避免重複，所以這邊先把基礎架構設定好

httpServers.ts

import { createServer } from "http";

export const http5000Server = createServer().listen(5000);
export const http5001Server = createServer().listen(5001);

index.ts

import { faviconListener } from "../listeners/faviconListener";
import { notFoundListener } from "../listeners/notFoundlistener";
import { http5000Server, http5001Server } from "./httpServers";

// 白頁，等等都透過 http://localhost:5000 的 F12 > Console
// 去發起 fetch 請求
http5000Server.on("request", function requestListener(req, res) {
  res.end();
  return;
});

// cross-origin resources
http5001Server.on("request", function requestListener(req, res) {
  if (req.url === "/favicon.ico") return faviconListener(req, res);
  // 等等的範例程式碼會放在這裡...
  return notFoundListener(req, res);
});

CORS Headers 整理

底下網羅 CORS 相關的 Headers，會在接下來的段落陸續介紹到

Simple Request

Simple Request，是指不觸發 Preflight Request 的請求，參考 MDN Web Docs，須滿足以下條件：

1. CORS-safelisted method => GET, HEAD, or POST
2. CORS-safelisted request-header => 可參考 CORS-safelisted request header & Access-Control-Allow-Headers
3. Content-Type 必須是 <form> 能設定的 enctype：application/x-www-form-urlencoded, multipart/form-data or text/plain => 因為 <form> 在 CORS 出現前，就能夠發起跨域請求，為了向後兼容性，避免舊網站的後端不支援 Preflight Request 而導致網站壞掉
4. 使用 XMLHTTPRequest，且沒有使用 xhr.upload.addEventListener()
5. Request 沒有使用 ReadableStream

補充 4,5 點（這兩個情境實務上比較少用），原因是 xhr.upload.addEventListener() 跟 ReadableStream 這兩個情況會讓 Request 本身變得不可控，瀏覽器沒辦法預先知道這個 Request 是否符合 1,2,3 點，所以只好觸發 Preflight Request

Simple Request 與 <form> 能發出的 Request 關係如下

Preflight Request + Redirection

實務上有多種情境會觸發 30x Redirect，例如


這時候 Preflight Request 會跟著導轉嗎？我們寫個測試案例

// preflight + redirect
if (req.url === "/old-path") {
  res.writeHead(308, { location: "http://localhost:5001/new-path" });
  res.end();
  return;
}
if (req.url === "/new-path") {
  if (req.method === "OPTIONS") {
    res.writeHead(204, {
      "access-control-allow-origin": "http://localhost:5000",
      "access-control-allow-methods": "PUT",
    });
    res.end();
    return;
  }
  res.writeHead(200, {
    "access-control-allow-origin": "http://localhost:5000",
    "access-control-allow-methods": "PUT",
    "content-type": "text/plain",
  });
  res.end();
  return;
}

瀏覽器打開 http://localhost:5000/ ，F12 > Console 執行以下程式碼

fetch("http://localhost:5001/old-path", { method: "PUT" });

結論是不行～

CORS-safelisted request header & Access-Control-Allow-Headers

參考 fetch.spec.whatwg.org，CORS-safelisted request header 如下：

• Accept
• Accept-Language
• Content-Language
• Content-Type
• Range

P.S. 針對以上 request headers 的 value，還有額外限制，有興趣的自行參考

Access-Control-Allow-Headers 實作

我們在 http5001Server 新增以下區塊

if (req.url === "/access-control-allow-headers") {
  if (req.method === "OPTIONS") {
    res.writeHead(204, {
      "access-control-allow-origin": "http://localhost:5000",
      "access-control-allow-headers": "authorization",
    });
    res.end();
    return;
  }
  res.writeHead(200, {
    "access-control-allow-origin": "http://localhost:5000",
    "access-control-allow-headers": "authorization",
  });
  res.end();
  return;
}

從 http://localhost:5000/ 的 F12 > Console 戳看看

fetch("http://localhost:5001/access-control-allow-headers", {
  headers: {
    authorization: "Basic 123",
  },
});

Preflight Request，這時候還不會帶上 authorization Request Header

Actual Request，這時候就會帶上 authorization Request Header

CORS-safelisted response header & Access-Control-Expose-Headers

參考 fetch.spec.whatwg.org，CORS-safelisted response header 代表預設能透過 JavaScript 讀取的 response headers，如下：

• Cache-Control
• Content-Language
• Content-Length
• Content-Type
• Expires
• Last-Modified
• Pragma

JavaScript Can Read All Same-Origin Response Headers

從 http://localhost:5000/ 的 F12 > Console 試試看戳自己，確實可以拿到所有 response headers

fetch("http://localhost:5000/").then(res => console.log(Object.fromEntries(res.headers.entries())))

// result
{
  "connection": "keep-alive",
  "content-length": "0",
  "date": "Mon, 07 Jul 2025 06:38:28 GMT",
  "keep-alive": "timeout=5"
}

CORS-safelisted response header 實測

接著在 http5001Server 新增以下區塊

if (req.url === "/cors-safelisted-response-header") {
  res.writeHead(200, {
    "access-control-allow-origin": "http://localhost:5000",
    "cache-control": "cache-control",
    "content-language": "content-language",
    "content-length": 0,
    "content-type": "text/html",
    expires: "expires",
    "last-modified": "last-modified",
    pragma: "pragma",
    "x-custom-header1": "x-custom-value1",
  });
  res.end();
  return;
}

從 http://localhost:5000/ 的 F12 > Console 戳看看，確實如同 spec 的描述，只能透過 JavaScript 讀取這些 Response Headers

fetch("http://localhost:5001/cors-safelisted-response-header").then(res => console.log(Object.fromEntries(res.headers.entries())))

// result
{
  "cache-control": "cache-control",
  "content-language": "content-language",
  "content-length": "0",
  "content-type": "text/html",
  "expires": "expires",
  "last-modified": "last-modified",
  "pragma": "pragma"
}

Access-Control-Expose-Headers 實作

使用 Access-Control-Expose-Headers，就可以增加 JavaScript 能讀取的 Response Headers

if (req.url === "/access-control-expose-headers") {
  res.writeHead(200, {
    "access-control-allow-origin": "http://localhost:5000",
    "cache-control": "cache-control",
    "content-language": "content-language",
    "content-length": 0,
    "content-type": "text/html",
    expires: "expires",
    "last-modified": "last-modified",
    pragma: "pragma",
    "access-control-expose-headers": "connection, date, keep-alive",
  });
  res.end();
  return;
}

從 http://localhost:5000/ 的 F12 > Console 戳看看，可以看到 connection, date, keep-alive 都可以讀取了～

fetch("http://localhost:5001/access-control-expose-headers").then(res => console.log(Object.fromEntries(res.headers.entries())))

// result
{
  "cache-control": "cache-control",
  "connection": "keep-alive",
  "content-language": "content-language",
  "content-length": "0",
  "content-type": "text/html",
  "date": "Mon, 07 Jul 2025 10:53:13 GMT",
  "expires": "expires",
  "keep-alive": "timeout=5",
  "last-modified": "last-modified",
  "pragma": "pragma"
}

小結

CORS 的概念很多，跟著我繼續探索下篇的內容吧～

參考資料

• https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CORS/Errors
• https://developer.mozilla.org/en-US/docs/Glossary/CORS
• https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request
• https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_request_header
• https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_response_header
• https://developer.mozilla.org/en-US/docs/Web/Security/Same-origin_policy
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Access-Control-Allow-Origin
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Access-Control-Allow-Headers
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Access-Control-Allow-Methods
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Access-Control-Allow-Credentials
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Access-Control-Max-Age
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Access-Control-Expose-Headers
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Access-Control-Request-Method
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Access-Control-Request-Headers