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:
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 options
The 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:asyncfunctionpostData(url ='', data ={}){// Default options are marked with *const response =awaitfetch(url,{method:'POST',// *GET, POST, PUT, DELETE, etc.mode:'cors',// no-cors, *cors, same-origincache:'no-cache',// *default, no-cache, reload, force-cache, only-if-cachedcredentials:'same-origin',// include, *same-origin, omitheaders:{'Content-Type':'application/json'// 'Content-Type': 'application/x-www-form-urlencoded',},redirect:'follow',// manual, *follow, errorreferrerPolicy:'no-referrer',// no-referrer, *no-referrer-when-downgrade, origin, origin-when-cross-origin, same-origin, strict-origin, strict-origin-when-cross-origin, unsafe-urlbody: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 included
To 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.
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 data
Use 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 file
Files can be uploaded using an HTML input element, FormData() and fetch().
The 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).
asyncfunction*makeTextFileLineIterator(fileURL){const utf8Decoder =newTextDecoder('utf-8');const response =awaitfetch(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 charyield chunk.substr(startIndex);}}asyncfunctionrun(){forawait(const line ofmakeTextFileLineIterator(urlOfFile)){processLine(line);}}run();
Checking that the fetch was successful
A 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){thrownewError('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 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:
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:
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')){thrownewTypeError("Oops, we haven't got JSON!");}return response.json();}).then((data)=>{/* process your data further */}).catch((error)=> console.error(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.modeno-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 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 ""), 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 =newBlob();addEventListener('fetch',(event)=>{// ServiceWorker intercepting a fetch
event.respondWith(newResponse(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.
Body
Both 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 literal
URLSearchParams
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 =newFormData(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 detection
Fetch 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?}