The Fetch API provides a JavaScript interface for accessing and manipulating parts of the protocol, such as requests and responses. It also provides a global fetch() method
that provides an easy, logical way to fetch resources asynchronously across the network. This kind of functionality was previously achieved using XMLHttpRequest . Fetch provides a better alternative that can be easily used by other technologies such as Service Workers . Fetch also provides a
single logical place to define other HTTP-related concepts such as CORS and extensions to HTTP. The fetch specification differs from jQuery.ajax() in the following significant ways: - The Promise returned from
fetch() won't reject on HTTP error status even if the response is an HTTP 404 or 500. Instead, as soon as the server responds with headers, the Promise will resolve normally
(with the ok property of the response set to false if the response isn't in the range 200–299), and it will only reject on network failure or if anything prevented the request from completing. - Unless
fetch() is called with the credentials option set to include , fetch() : - won't
send cookies in cross-origin requests
- won't set any cookies sent back in cross-origin responses
- As of August 2018, the default credentials policy changed to same-origin. Firefox was also modified in version 61.0b13)
A basic fetch request is really simple to set up. Have a look at the following code: fetch('http://example.com/movies.json')
.then((response) => response.json())
.then((data) => console.log(data));
Here we are fetching a JSON file across the network and printing it to the console. The simplest use of fetch() takes one argument — the
path to the resource you want to fetch — and does not directly return the JSON response body but instead returns a promise that resolves with a Response object. The Response object, in turn, does not directly contain the actual JSON response body but is instead a representation of the entire HTTP response. So, to
extract the JSON body content from the Response object, we use the json() method, which returns a second promise that resolves with the result of parsing the response body text as JSON. Note: See the
Body section for similar methods to extract other types of body content. Fetch requests are controlled by the connect-src directive of Content Security Policy rather than the directive of the resources it's retrieving. Supplying request optionsThe fetch() method can optionally accept a second parameter, an init object that allows you to control a number of different settings: See
fetch() for the full options available, and more details. // Example POST method implementation:
async function postData(url = '', data = {}) {
// Default options are marked with *
const response = await fetch(url, {
method: 'POST', // *GET, POST, PUT, DELETE, etc.
mode: 'cors', // no-cors, *cors, same-origin
cache: 'no-cache', // *default, no-cache, reload, force-cache, only-if-cached
credentials: 'same-origin', // include, *same-origin, omit
headers: {
'Content-Type': 'application/json'
// 'Content-Type': 'application/x-www-form-urlencoded',
},
redirect: 'follow', // manual, *follow, error
referrerPolicy: 'no-referrer', // no-referrer, *no-referrer-when-downgrade, origin, origin-when-cross-origin, same-origin, strict-origin, strict-origin-when-cross-origin, unsafe-url
body: JSON.stringify(data) // body data type must match "Content-Type" header
});
return response.json(); // parses JSON response into native JavaScript objects
}
postData('https://example.com/answer', { answer: 42 })
.then((data) => {
console.log(data); // JSON data parsed by `data.json()` call
});
Note that mode: "no-cors" only allows a limited set of headers in the request: Accept Accept-Language Content-Language Content-Type with a value of application/x-www-form-urlencoded , multipart/form-data , or text/plain
Sending a request with credentials includedTo cause browsers to send a request with credentials included on both same-origin and cross-origin calls, add credentials: 'include' to the init object you pass to the fetch() method. fetch('https://example.com', {
credentials: 'include'
});
Note: Access-Control-Allow-Origin is prohibited from using a wildcard for requests with credentials: 'include' . In such cases, the exact origin must be provided; even if you are using a CORS unblocker extension, the requests will still fail. Note: Browsers should not send credentials in preflight requests irrespective of this setting. For more information see: CORS
> Requests with credentials. If you only want to send credentials if the request URL is on the same origin as the calling script, add credentials: 'same-origin' . // The calling script is on the origin 'https://example.com'
fetch('https://example.com', {
credentials: 'same-origin'
});
To instead ensure browsers don't include credentials in the request, use credentials: 'omit' . fetch('https://example.com', {
credentials: 'omit'
})
Uploading JSON dataUse fetch() to POST JSON-encoded data. const data = { username: 'example' };
fetch('https://example.com/profile', {
method: 'POST', // or 'PUT'
headers: {
'Content-Type': 'application/json',
},
body: JSON.stringify(data),
})
.then((response) => response.json())
.then((data) => {
console.log('Success:', data);
})
.catch((error) => {
console.error('Error:', error);
});
Uploading a fileFiles can be uploaded using an HTML <input type="file" /> input element, FormData() and fetch() . const formData = new FormData();
const 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())
.then((result) => {
console.log('Success:', result);
})
.catch((error) => {
console.error('Error:', error);
});
Uploading multiple filesFiles can be uploaded using an HTML <input type="file" multiple /> input element, FormData() and
fetch() . const formData = new FormData();
const photos = document.querySelector('input[type="file"][multiple]');
formData.append('title', 'My Vegas Vacation');
let i = 0;
for (const photo of photos.files) {
formData.append(`photos_${i}`, photo);
i++;
}
fetch('https://example.com/posts', {
method: 'POST',
body: formData,
})
.then((response) => response.json())
.then((result) => {
console.log('Success:', result);
})
.catch((error) => {
console.error('Error:', error);
});
Processing a text file line by lineThe chunks that
are read from a response are not broken neatly at line boundaries and are Uint8Arrays, not strings. If you want to fetch a text file and process it line by line, it is up to you to handle these complications. The following example shows one way to do this by creating a line iterator (for simplicity, it assumes the text is UTF-8, and doesn't handle fetch errors). async function* makeTextFileLineIterator(fileURL) {
const utf8Decoder = new TextDecoder('utf-8');
const response = await fetch(fileURL);
const reader = response.body.getReader();
let { value: chunk, done: readerDone } = await reader.read();
chunk = chunk ? utf8Decoder.decode(chunk) : '';
const re = /\n|\r|\r\n/gm;
let startIndex = 0;
let result;
while (true) {
let result = re.exec(chunk);
if (!result) {
if (readerDone) break;
let remainder = chunk.substr(startIndex);
({ value: chunk, done: readerDone } = await reader.read());
chunk = remainder + (chunk ? utf8Decoder.decode(chunk) : '');
startIndex = re.lastIndex = 0;
continue;
}
yield chunk.substring(startIndex, result.index);
startIndex = re.lastIndex;
}
if (startIndex < chunk.length) {
// Last line didn't end in a newline char
yield chunk.substr(startIndex);
}
}
async function run() {
for await (const line of makeTextFileLineIterator(urlOfFile)) {
processLine(line);
}
}
run();
Checking that the fetch was successfulA fetch() promise will reject with a
TypeError when a network error is encountered or CORS is misconfigured on the server-side, although this usually means permission issues or similar — a 404 does not constitute a network error, for example. An accurate check for a successful fetch() would include checking that the promise resolved, then checking that the
Response.ok property has a value of true. The code would look something like this: fetch('flowers.jpg')
.then((response) => {
if (!response.ok) {
throw new Error('Network response was not OK');
}
return response.blob();
})
.then((myBlob) => {
myImage.src = URL.createObjectURL(myBlob);
})
.catch((error) => {
console.error('There has been a problem with your fetch operation:', error);
});
Supplying your own request objectInstead 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: const myHeaders = new Headers();
const myRequest = new Request('flowers.jpg', {
method: 'GET',
headers: myHeaders,
mode: 'cors',
cache: 'default',
});
fetch(myRequest)
.then((response) => response.blob())
.then((myBlob) => {
myImage.src = URL.createObjectURL(myBlob);
});
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:
const anotherRequest = new Request(myRequest, myInit);
This is pretty useful, as request and response bodies can only be used once. Making a copy like this allows you to effectively use the request/response again while varying the init options if desired. The copy must be made before the body is read. 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: const content = 'Hello World';
const 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: const 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']
myHeaders.delete('X-Custom-Header');
console.log(myHeaders.get('X-Custom-Header')); // null
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: const 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: fetch(myRequest)
.then((response) => {
const contentType = response.headers.get('content-type');
if (!contentType || !contentType.includes('application/json')) {
throw new TypeError("Oops, we haven't got JSON!");
}
return response.json();
})
.then((data) => {
/* process your data further */
})
.catch((error) => console.error(error));
GuardSince 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 object obtained from a response
(Response.headers ).immutable : guard that renders a headers object read-only; mostly used for ServiceWorkers.
Note: You may not append or set the Content-Length header on a guarded headers object for a response . Similarly, inserting Set-Cookie into a response header is not allowed: ServiceWorkers are not allowed to set cookies via synthesized responses. Response objectsAs 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 ""), which corresponds to the HTTP status code message. Note that HTTP/2 does not
support status messages.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 value.
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: const myBody = new Blob();
addEventListener('fetch', (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.) Note: The static method
error() 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. BodyBoth requests and responses may contain body data. A body is an instance of any of the following types: ArrayBuffer TypedArray (Uint8Array and friends)DataView Blob File String , or a string literalURLSearchParams FormData
The Request and
Response interfaces share the following methods to extract a body. These all return a promise that is eventually resolved with the actual content. Request.arrayBuffer() / Response.arrayBuffer() Request.blob() / Response.blob() Request.formData() / Response.formData() Request.json() / Response.json() Request.text() / Response.text()
This makes usage of non-textual data much easier than it was with XHR.
Request bodies can be set by passing body parameters: const 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. Feature detectionFetch API support can be detected by checking for the existence of Headers , Request , Response or
fetch() on the Window or Worker scope. For example: if (window.fetch) {
// run my fetch request here
} else {
// do something with XMLHttpRequest?
}
Specifications
Specification |
---|
Fetch Standard # fetch-method
|
Browser compatibilityBCD tables only load in the browser
How do you fetch data from JavaScript?
The Fetch API allows you to asynchronously request for a resource. Use the fetch() method to return a promise that resolves into a Response object. To get the actual data, you call one of the methods of the Response object e.g., text() or json() . These methods resolve into the actual data.
Can we use fetch in JavaScript?
JavaScript | fetch() Method. The fetch() method in JavaScript is used to request to the server and load the information on the webpages. The request can be of any APIs that return the data of the format JSON or XML. This method returns a promise.
How does fetch works in JavaScript?
The fetch() method starts the process of fetching a resource from a server. The fetch() method returns a Promise that resolves to a Response object. 😀 No need for XMLHttpRequest anymore.
How get JSON data from Fetch?
GET JSON data
loadNames(); await fetch('/api/names') starts a GET request, and evaluates to the response object when the request is complete. Then, from the server response, you can parse the JSON into a plain JavaScript object using await response.
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