JavaScript can send network requests to the server and load new information whenever it’s needed.
For example, we can use a network request to:
Please note, if the request body is a string, then Content-Type header is set to text/plain;charset=UTF-8 by default. But, as we’re going to send JSON, we use headers option to send application/json instead, the correct Content-Type for JSON-encoded data. 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. Freight 500/1500 Freight500 and Freight1500 provide industry-leading payload capacity.Fetch Robotics offers two sizes of industrial Autonomous Mobile Robots (AMRs) to enable on-demand pallet transport in your facility with no infrastructure changes required.
- Submit an order,
- Load user information,
- Receive latest updates from the server,
- …etc.
…And all of that without reloading the page!
There’s an umbrella term “AJAX” (abbreviated Asynchronous JavaScript And XML) for network requests from JavaScript. We don’t have to use XML though: the term comes from old times, that’s why that word is there. You may have heard that term already.
There are multiple ways to send a network request and get information from the server.
The
fetch()
method is modern and versatile, so we’ll start with it. It’s not supported by old browsers (can be polyfilled), but very well supported among the modern ones.The basic syntax is:
url
– the URL to access.options
– optional parameters: method, headers etc.
Without
options
, that is a simple GET request, downloading the contents of the url
.The browser starts the request right away and returns a promise that the calling code should use to get the result.
Getting a response is usually a two-stage process.
First, the
promise
, returned by fetch
, resolves with an object of the built-in Response class as soon as the server responds with headers.At this stage we can check HTTP status, to see whether it is successful or not, check headers, but don’t have the body yet.
The promise rejects if the
fetch
was unable to make HTTP-request, e.g. network problems, or there’s no such site. Abnormal HTTP-statuses, such as 404 or 500 do not cause an error.We can see HTTP-status in response properties:
status
– HTTP status code, e.g. 200.ok
– boolean,true
if the HTTP status code is 200-299.
For example:
Second, to get the response body, we need to use an additional method call.
Response
provides multiple promise-based methods to access the body in various formats:Fetch Post 500 Error
response.text()
– read the response and return as text,response.json()
– parse the response as JSON,response.formData()
– return the response asFormData
object (explained in the next chapter),response.blob()
– return the response as Blob (binary data with type),response.arrayBuffer()
– return the response as ArrayBuffer (low-level representation of binary data),- additionally,
response.body
is a ReadableStream object, it allows you to read the body chunk-by-chunk, we’ll see an example later.
For instance, let’s get a JSON-object with latest commits from GitHub:
Or, the same without
await
, using pure promises syntax: Keyboard maestro 8 2 – hot key tasking solution 9.To get the response text,
await response.text()
instead of .json()
:As a show-case for reading in binary format, let’s fetch and show a logo image of “fetch” specification (see chapter Blob for details about operations on
Blob
):We can choose only one body-reading method.
If we’ve already got the response with
response.text()
, then response.json()
won’t work, as the body content has already been processed.Response headers
The response headers are available in a Map-like headers object in
response.headers
.It’s not exactly a Map, but it has similar methods to get individual headers by name or iterate over them:
Request headers
To set a request header in
fetch
, we can use the headers
option. It has an object with outgoing headers, like this:…But there’s a list of forbidden HTTP headers that we can’t set:
Accept-Charset
,Accept-Encoding
Access-Control-Request-Headers
Access-Control-Request-Method
Connection
Content-Length
Cookie
,Cookie2
Date
DNT
Expect
Host
Keep-Alive
Origin
Referer
TE
Trailer
Transfer-Encoding
Upgrade
Via
Proxy-*
Sec-*
These headers ensure proper and safe HTTP, so they are controlled exclusively by the browser.
POST requests
![Fetch 500 Fetch 500](https://cdn10.bigcommerce.com/s-gyhhtwx4/products/3022/images/7291/079346031160_front_1500__76012.1582553925.500.659.jpg?c=2)
To make a
POST
request, or a request with another method, we need to use fetch
options:method
– HTTP-method, e.g.POST
,body
– the request body, one of:- a string (e.g. JSON-encoded),
FormData
object, to submit the data asform/multipart
,Blob
/BufferSource
to send binary data,- URLSearchParams, to submit the data in
x-www-form-urlencoded
encoding, rarely used.
The JSON format is used most of the time.
Privatus 5 0 6 – automated privacy protection. For example, this code submits
user
object as JSON:Please note, if the request
body
is a string, then Content-Type
header is set to text/plain;charset=UTF-8
by default.Fetch First 500 Rows Only
But, as we’re going to send JSON, we use
headers
option to send application/json
instead, the correct Content-Type
Cyberlink makeupdirector ultra 2 0 2105 64703. for JSON-encoded data.Sending an image
We can also submit binary data with
fetch
using Blob
or BufferSource
objects.In this example, there’s a
<canvas>
where we can draw by moving a mouse over it. A click on the “submit” button sends the image to the server:Please note, here we don’t set
Content-Type
header manually, because a Blob
object has a built-in type (here image/png
, as generated by toBlob
). For Blob
objects that type becomes the value of Content-Type
.The
submit()
function can be rewritten without async/await
like this:Summary
A typical fetch request consists of two
await
calls:Or, without
await
:Response properties:
response.status
– HTTP code of the response,response.ok
–true
is the status is 200-299.response.headers
– Map-like object with HTTP headers.
Methods to get response body:
response.text()
– return the response as text,response.json()
– parse the response as JSON object,response.formData()
– return the response asFormData
object (form/multipart encoding, see the next chapter),response.blob()
– return the response as Blob (binary data with type),response.arrayBuffer()
– return the response as ArrayBuffer (low-level binary data),
Fetch 500 Error
Fetch options so far:
method
– HTTP-method,headers
– an object with request headers (not any header is allowed),body
– the data to send (request body) asstring
,FormData
,BufferSource
,Blob
orUrlSearchParams
object.
Fetch 500 Piece Jigsaw Puzzle
In the next chapters we’ll see more options and use cases of
fetch
.