Using Fetch

Fetch API 提供了一種 JavaScript Interface 來操作 HTTP pipeline,比方 request 和 response。同時它也提供了 global 的 fetch() method,使得在網路上非同步地 fetch resources 這件事變得簡單易懂。

同樣的功能,以前都是使用 XMLHttpRequest,而 Fetch 作為其替代方案,能更方便地整合在如 Service Workers 等相關技術上。此外,Fetch 具備額外的 logical palce,能拿來定義其他和 HTTP 有關的東西,像是 CORS 和 HTTP extensions。

 fetch 和 jQuery.ajax() 有三個主要的差異:

  • fetch() 回傳的 promise 不會 reject HTTP 的 error status,就算是 HTTP 404 或 500 也一樣。相反地,它會正常地 resolve,並把 ok status 設為 false。會讓它發生 reject 的只有網路錯誤或其他會中斷 request 的情況。
  • fetch 可以接收跨站的 cookies,你可以用 Fetch 來建立跨站的 session。
  • fetch 不會傳送 cookies,除非你有設定 credentials 的 init option。 (Since Aug 25, 2017. The spec changed the default credentials policy to same-origin. Firefox changed since 61.0b13.)

使用 Fetch 發送請求 ( request )


  .then(function(response) {
    return response.json();
  .then(function(myJson) {

這裡要使用 fetch 透過網路取得 json 然後印出在 console,最簡單的方式只需要一個參數就是資料的 URI,fetch 會回傳一個包含 response 的 promise 。

這個範例使用的 url 只是示意用。

回傳的 response 需要透過 json() (在 Body 可以找到定義, Body 是用 RequestResponse 實作出來的物件.)

備註: 其實 Body 還提供了其他類似的功能可以將內容輸成其他類型格式,詳見Body 

Fetch 請求的安全性 Content Security Policy(內容安全策略) 是由 header 中的 connect-src directive 所設定 ,並非其他 directive ( 比如:img-src、default-src 等)。

Request 可用的設定值

fetch() 第二個參數是選用的,可以傳送一個 init Object 來設定 request。

更多可以用的設定值詳見 fetch() 

// 來發個 POST Request:

postData('', {answer: 42})
  .then(data => console.log(data)) // JSON from `response.json()` call
  .catch(error => console.error(error))

function postData(url, data) {
  // Default options are marked with *
  return fetch(url, {
    body: JSON.stringify(data), // must match 'Content-Type' header
    cache: 'no-cache', // *default, no-cache, reload, force-cache, only-if-cached
    credentials: 'same-origin', // include, same-origin, *omit
    headers: {
      'user-agent': 'Mozilla/4.0 MDN Example',
      'content-type': 'application/json'
    method: 'POST', // *GET, POST, PUT, DELETE, etc.
    mode: 'cors', // no-cors, cors, *same-origin
    redirect: 'follow', // manual, *follow, error
    referrer: 'no-referrer', // *client, no-referrer
  .then(response => response.json()) // 輸出成 json

包含憑證(Credentials) 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' 

fetch('', {
  credentials: 'include'

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

// The calling script is on the origin ''

fetch('', {
  credentials: 'same-origin'

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

fetch('', {
  credentials: 'omit'


使用 fetch() 來 POST JSON 格式的資料。

var url = '';
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));


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

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

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

fetch('', {
  method: 'PUT',
  body: formData
.then(response => response.json())
.catch(error => console.error('Error:', error))
.then(response => console.log('Success:', response));


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:

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:

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.

Note: 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.


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:

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:

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

The contents can be queried and retrieved:

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"]

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:

var myResponse = Response.error();
try {
  myResponse.headers.set("Origin", "");
} 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:

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); });


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.

Note: 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:

var myBody = new Blob();

addEventListener('fetch', function(event) { // ServiceWorker intercepting a fetch
    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.)

Note: 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.


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

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:

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.


想確認是否支持 Fetch API,可透過檢查 HeadersRequestResponsefetch() 是否存在 WindowWorker 域中。例如:

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


在不支援 Fetch 的瀏覽器, 可改用 Fetch Polyfill 來重新支持缺少的 fetch 功能。


技術名稱 狀態 說明
Fetch Living Standard Initial definition


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