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使用 Fetch API

Fetch API 提供一個用來發送 HTTP 請求與處理回應的 JavaScript 介面。

Fetch 是 XMLHttpRequest 的現代替代方案：與使用回呼函式的 XMLHttpRequest 不同，Fetch 基於 Promise，而且能夠與 service worker 和跨來源資源共享（CORS）等特性整合。

使用 Fetch API，呼叫 fetch() 就可以建立請求，它在 window 和 worker 的上下文中都能作為全域函式使用。你可以傳入一個 Request 物件，或是一個 URL 字串，以及額外的參數設定來使用 fetch。

fetch() 函式回傳一個 Promise，當伺服器回應時會兌現一個代表伺服器回應的 Response 物件。接著你可以藉由呼叫物件上合適的方法，來檢查回應狀態，並且以各種格式擷取回應內容，包括文字檔與 JSON。

以下是一個使用 fetch() 來從伺服器獲取一些 JSON 資料的最小函數：

js
async function getData() {
  const url = "https://example.org/products.json";
  try {
    const response = await fetch(url);
    if (!response.ok) {
      throw new Error(`回應狀態：${response.status}`);
    }

    const result = await response.json();
    console.log(result);
  } catch (error) {
    console.error(error.message);
  }
}

我們宣告一個 URL 字串，並且將其作為參數傳入 fetch()，並且不帶額外選項。

當發生一些錯誤，fetch() 函式會以錯誤拒絕這個 promise，不過如果是伺服器回應錯誤狀態，例如 404，這並不會：因此我們應該主動檢查回應的狀態，如果回應狀態不正確則拋出錯誤。

否則，我們可以藉由呼叫 Response 的 json() 方法來以 JSON 獲取回應的主體，並記錄其中的值。要注意就像 fetch() 函式一樣，json() 也是非同步的，而且所有其他用來獲取回應主體的方法也都一樣。

在本頁接下來的部分，我們將更詳細的探討這個過程的各階段。

建立請求

你可以呼叫 fetch() 來建立請求，傳入：

欲請求的資源定義，這可以是：
- 一個包含 URL 字串
- 一個物件，例如 URL 的實例，其字串化器可以產生包含 URL 的字串
- 一個 Request 實例
可選的物件，包含可以用來設定請求的選項

在這個章節我們我會看看一些最常用的選項。如果你想要查看所有可用的選項，你可以查看 fetch() 參考頁。

設定請求方法

預設下，fetch 會做 GET 請求，但你可以使用 method 這個選項來使用不同的請求方法：

js
const response = await fetch("https://example.org/post", {
  method: "POST",
  // …
});

如果 mode 設為 no-cors，則 method 一定要是 GET、POST、HEAD 其中一個。

設定標頭

標頭會提供伺服器有關請求的相關資訊，舉例來說，在 POST 請求中，Content-Type 這個標頭會告訴伺服器請求主體的格式。

你可以在 header 傳入物件實字，包含 header-name: header-value 的屬性：

js
const response = await fetch("https://example.org/post", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
  },
  body: JSON.stringify({ username: "example" }),
  // …
});

另外，你也可以建立一個 Headers 物件，使用 Headers.append() 就可以新增標頭，然後將 Headers 物件賦值給 header 選項：

js
const myHeaders = new Headers();
myHeaders.append("Content-Type", "application/json");

const response = await fetch("https://example.org/post", {
  method: "POST",
  headers: myHeaders,
  body: JSON.stringify({ username: "example" }),
  // …
});

相較於一般的物件，Headers 物件提供了一些額外的輸入淨化。例如，標頭的名稱會變成小寫，標頭值前後的空白字符會被去除，也避免一些特定標頭的設定。許多標頭是自動被瀏覽器設定的，腳本被禁止設定，如禁止的請求標頭。如果 mode 選項被設定為 no-cors，則允許的標頭集將進一步受到限制。

在 GET 請求中發送資料

GET 請求並沒有主體，但你仍然可以透過將資料作為查詢字串附加到 URL 來向伺服器發送資料。這是向伺服器發送表單資料的常用方法。你可以使用 URLSearchParams 來編碼這個資料，然後將其附加到 URL：

js
const params = new URLSearchParams();
params.append("username", "example");

// GET 請求會送到 https://example.org/login?username=example
const response = await fetch(`https://example.org/login?${params}`);

包含憑證的 Request 用法

要讓瀏覽器將 credentials 跟著 request 一起送出, 方式就是在 init object 加上 credentials: 'include'

js
fetch("https://example.com", {
  credentials: "include",
});

如果只想要把 credentials 發送給同源的 URL ，加上credentials: 'same-origin'。

js
// The calling script is on the origin 'https://example.com'

fetch("https://example.com", {
  credentials: "same-origin",
});

或要確保瀏覽器不會帶著 credentials 請求，可以用 credentials: 'omit' 。

js
fetch("https://example.com", {
  credentials: "omit",
});

上傳 JSON 資料

使用 fetch() 來 POST JSON 格式的資料。

js
var url = "https://example.com/profile";
var data = { username: "example" };

fetch(url, {
  method: "POST", // or 'PUT'
  body: JSON.stringify(data), // data can be `string` or {object}!
  headers: new Headers({
    "Content-Type": "application/json",
  }),
})
  .then((res) => res.json())
  .catch((error) => console.error("Error:", error))
  .then((response) => console.log("Success:", response));

上傳檔案

上傳檔案可以透過使用 HTML <input type="file" /> input element, FormData() 與fetch().

js
var formData = new FormData();
var fileField = document.querySelector("input[type='file']");

formData.append("username", "abc123");
formData.append("avatar", fileField.files[0]);

fetch("https://example.com/profile/avatar", {
  method: "PUT",
  body: formData,
})
  .then((response) => response.json())
  .catch((error) => console.error("Error:", error))
  .then((response) => console.log("Success:", response));

如何確認 fetch 是否成功

當fetch()遇到 CORS 或 server 設定錯誤導致 network error 時, promise 會 reject 並附上TypeError的回應, 但在權限或類似問題導致 404 的常見狀況下, 卻不會導致 network error.

因此, 確認fetch()是否成功的正確方式, 應包含檢查 promise resolved, 以及檢查Response.ok的屬性是否為 true. 代碼如下例：

js
fetch("flowers.jpg")
  .then(function (response) {
    if (response.ok) {
      return response.blob();
    }
    throw new Error("Network response was not ok.");
  })
  .then(function (myBlob) {
    var objectURL = URL.createObjectURL(myBlob);
    myImage.src = objectURL;
  })
  .catch(function (error) {
    console.log(
      "There has been a problem with your fetch operation: ",
      error.message,
    );
  });

Supplying your own request object

Instead of passing a path to the resource you want to request into the fetch() call, you can create a request object using the Request() constructor, and pass that in as a fetch() method argument:

js
var myHeaders = new Headers();

var myInit = {
  method: "GET",
  headers: myHeaders,
  mode: "cors",
  cache: "default",
};

var myRequest = new Request("flowers.jpg", myInit);

fetch(myRequest)
  .then(function (response) {
    return response.blob();
  })
  .then(function (myBlob) {
    var objectURL = URL.createObjectURL(myBlob);
    myImage.src = objectURL;
  });

Request() accepts exactly the same parameters as the fetch() method. You can even pass in an existing request object to create a copy of it:

var anotherRequest = new Request(myRequest, myInit);

This is pretty useful, as request and response bodies are one use only. Making a copy like this allows you to make use of the request/response again, while varying the init options if desired. The copy must be made before the body is read, and reading the body in the copy will also mark it as read in the original request.

備註：There is also a clone() method that creates a copy. Both methods of creating a copy will fail if the body of the original request or response has already been read, but reading the body of a cloned response or request will not cause it to be marked as read in the original.

Headers

The Headers interface allows you to create your own headers object via the Headers() constructor. A headers object is a simple multi-map of names to values:

js
var content = "Hello World";
var myHeaders = new Headers();
myHeaders.append("Content-Type", "text/plain");
myHeaders.append("Content-Length", content.length.toString());
myHeaders.append("X-Custom-Header", "ProcessThisImmediately");

The same can be achieved by passing an array of arrays or an object literal to the constructor:

js
myHeaders = new Headers({
  "Content-Type": "text/plain",
  "Content-Length": content.length.toString(),
  "X-Custom-Header": "ProcessThisImmediately",
});

The contents can be queried and retrieved:

js
console.log(myHeaders.has("Content-Type")); // true
console.log(myHeaders.has("Set-Cookie")); // false
myHeaders.set("Content-Type", "text/html");
myHeaders.append("X-Custom-Header", "AnotherValue");

console.log(myHeaders.get("Content-Length")); // 11
console.log(myHeaders.get("X-Custom-Header")); // ["ProcessThisImmediately", "AnotherValue"]

myHeaders.delete("X-Custom-Header");
console.log(myHeaders.get("X-Custom-Header")); // [ ]

Some of these operations are only useful in ServiceWorkers, but they provide a much nicer API for manipulating headers.

All of the Headers methods throw a TypeError if a header name is used that is not a valid HTTP Header name. The mutation operations will throw a TypeError if there is an immutable guard (see below). Otherwise they fail silently. For example:

js
var myResponse = Response.error();
try {
  myResponse.headers.set("Origin", "http://mybank.com");
} catch (e) {
  console.log("Cannot pretend to be a bank!");
}

A good use case for headers is checking whether the content type is correct before you process it further. For example:

js
fetch(myRequest)
  .then(function (response) {
    var contentType = response.headers.get("content-type");
    if (contentType && contentType.includes("application/json")) {
      return response.json();
    }
    throw new TypeError("Oops, we haven't got JSON!");
  })
  .then(function (json) {
    /* process your JSON further */
  })
  .catch(function (error) {
    console.log(error);
  });

Guard

Since headers can be sent in requests and received in responses, and have various limitations about what information can and should be mutable, headers objects have a guard property. This is not exposed to the Web, but it affects which mutation operations are allowed on the headers object.

Possible guard values are:

none: default.
request: guard for a headers object obtained from a request (Request.headers).
request-no-cors: guard for a headers object obtained from a request created with Request.mode no-cors.
response: guard for a Headers obtained from a response (Response.headers).
immutable: Mostly used for ServiceWorkers; renders a headers object read-only.

備註：You may not append or set a request guarded Headers' Content-Length header. Similarly, inserting Set-Cookie into a response header is not allowed: ServiceWorkers are not allowed to set cookies via synthesized responses.

Response objects

As you have seen above, Response instances are returned when fetch() promises are resolved.

The most common response properties you'll use are:

Response.status — An integer (default value 200) containing the response status code.
Response.statusText — A string (default value "OK"), which corresponds to the HTTP status code message.
Response.ok — seen in use above, this is a shorthand for checking that status is in the range 200-299 inclusive. This returns a Boolean.

They can also be created programmatically via JavaScript, but this is only really useful in ServiceWorkers, when you are providing a custom response to a received request using a respondWith() method:

js
var myBody = new Blob();

addEventListener("fetch", function (event) {
  // ServiceWorker intercepting a fetch
  event.respondWith(
    new Response(myBody, {
      headers: { "Content-Type": "text/plain" },
    }),
  );
});

The Response() constructor takes two optional arguments — a body for the response, and an init object (similar to the one that Request() accepts.)

備註：The static method error() simply returns an error response. Similarly, redirect() returns a response resulting in a redirect to a specified URL. These are also only relevant to Service Workers.

Body

Both requests and responses may contain body data. A body is an instance of any of the following types:

ArrayBuffer
ArrayBufferView (Uint8Array and friends)
Blob/File
string
URLSearchParams
FormData

The Body mixin defines the following methods to extract a body (implemented by both Request and Response). These all return a promise that is eventually resolved with the actual content.

This makes usage of non-textual data much easier than it was with XHR.

Request bodies can be set by passing body parameters:

js
var form = new FormData(document.getElementById("login-form"));
fetch("/login", {
  method: "POST",
  body: form,
});

Both request and response (and by extension the fetch() function), will try to intelligently determine the content type. A request will also automatically set a Content-Type header if none is set in the dictionary.

特性偵測

想確認是否支持 Fetch API，可透過檢查 Headers、Request、Response 或 fetch() 是否存在 Window 或 Worker 域中。例如：

js
if (self.fetch) {
  // run my fetch request here
} else {
  // do something with XMLHttpRequest?
}

參見

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This page was last modified on 2026年4月21日 by MDN contributors.

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Fetch API
指南
1. 使用 Fetch API
2. Using Deferred Fetch
介面
方法