Deep Insights Into JavaScript’s Fetch API | by Sabesan Sathananthan

Synchronous properties of the Response object

After the fetch() request is profitable, you get a Response object. It corresponds to the HTTP response of the server.

const response = await fetch(url);

As talked about earlier, the information contained in Response is learn asynchronously by way of the Stream interface, however it additionally incorporates some synchronous attributes, which correspond to the header data of the HTTP response (Headers), which might be learn instantly.

Within the above instance, response.standing and response.statusText are the synchronous attributes of Response and might be learn instantly.


The Response.okay property returns a boolean worth, indicating whether or not the request is profitable, true corresponds to the HTTP request standing code 200 to 299, and false corresponds to different standing codes.


The Response.standing property returns a quantity indicating the standing code of the HTTP response (for instance, 200, indicating a profitable request).


The Response.statusText property returns a string representing the standing data of the HTTP response (for instance, after the request is profitable, the server returns “OK”).


The Response.url property returns the requested URL. If the URL has a redirect, this attribute returns the ultimate URL.


The Response.sort property returns the kind of request. The attainable values ​​are as follows:

  • primary: Extraordinary, same-origin request.
  • cors: Cross-origin request.
  • error: Community errors, primarily used for service employees.
  • opaque: If the mode attribute of the fetch() request is about to no-cors, this response worth will likely be returned.
  • opaqueredirect: If the redirect attribute of the fetch() request is about to handbook, this response worth will likely be returned.


The Response.redirected property returns a Boolean worth, indicating whether or not the request has been redirected.

Decide whether or not the request is profitable

After fetch() sends a request, there is a crucial level to notice: fetch() will report an error solely when there’s a community error or can not join. In different instances, no error will likely be reported, however the request is taken into account profitable.

This implies, even when the standing code returned by the server is 4xx or 5xx, fetch() won’t report an error (i.e. The Promise won’t change into rejected). Solely by acquiring the true standing code of the HTTP response by way of the Responese.standing property, can or not it’s decided whether or not the request is profitable. Please see the next instance:

Within the above instance, the Responese.standing attribute should be equal to 2xx (200~299) to find out that the request is profitable. There’s no want to think about the URL soar (standing code is 3xx) as a result of fetch() will robotically convert the jumped standing code to 200. One other technique is to find out whether or not Responese.okay is true.

Response.headers property

The Response object additionally has a Responese.headers property, which factors to a Headers object, which corresponds to all of the headers of the HTTP response. Headers objects might be traversed utilizing for...of loops.

The Headers object gives the next strategies to govern headers.

  • Headers.get(): In response to the required key identify, return the key-value.
  • Headers.has(): Returns a Boolean worth indicating whether or not a header is included.
  • Headers.set(): Set the required key identify as the brand new key-value, if the important thing identify doesn’t exist, it is going to be added.
  • Headers.append(): Add headers.
  • Headers.delete(): Delete the header.
  • Headers.keys(): Return an iterator that may traverse all of the keys in flip.
  • Headers.values(): Return an iterator that may traverse all key values ​​in flip.
  • Headers.entries(): Return an iterator that may traverse all key-value pairs in flip ([key, value]).
  • Headers.forEach(): Traverse the headers, in flip. Every header will execute a parameter operate.

A few of the above strategies can modify the headers as a result of they inherit from the Headers interface. For HTTP responses, modifying headers is of little significance — many headers are read-only and browsers don’t enable modification. Amongst these strategies, probably the most generally used is response.headers.get(), which is used to learn the worth of a sure header.

The Headers.keys() and Headers.values() strategies are used to traverse the header keys and key values ​​respectively.

The Headers.forEach() technique also can traverse all key values ​​and key names.

Find out how to learn content material

The Response object gives totally different studying strategies based on various kinds of information returned by the server.

  • response.textual content(): Get the textual content string.
  • response.json(): Get the JSON object.
  • response.blob(): Get the binary Blob object.
  • response.formData(): Get the FormData object.
  • response.arrayBuffer(): Get the binary ArrayBuffer object.

The above 5 studying strategies are all asynchronous and all return Promise objects. You should wait till the tip of the asynchronous operation to get the whole information returned by the server.

response.textual content()

response.textual content() can be utilized to get textual content information, reminiscent of HTML information.


response.json() is principally used to get the JSON information returned by the server. The instance has been given earlier.


response.formData() is principally utilized in Service Employee to intercept the shape submitted by the person, modify some information, after which submit it to the server.


response.blob() is used to get the binary file.

The above instance reads the flower.jpg picture file and shows it on the internet web page.


response.arrayBuffer() is principally used to acquire streaming media information.

The above instance is an instance the place response.arrayBuffer() will get the audio file tune.ogg after which performs it on-line.


The Stream object can solely be learn as soon as and it’s gone after studying. Which means that solely one of many 5 studying strategies within the earlier part can be utilized, in any other case, an error will likely be reported.

let textual content =  await response.textual content();
let json = await response.json(); // Report an error

The above instance makes use of response.textual content() first after which reads the Stream. After calling response.json() later, there’s no content material to learn, so an error is reported. The Response object gives the response.clone() technique, which creates a duplicate of the Response object and implements a number of reads.

Within the above instance, response.clone() made a duplicate of the Response object after which learn the identical picture twice. The Response object additionally has a Response.redirect() technique, which is used to redirect the Response end result to the required URL. This technique is mostly solely utilized in Service Employee, so I received’t introduce it right here.

Response.physique attribute

The Response.physique property is the underlying interface uncovered by the Response object. It returns a ReadableStream object for person operations. It may be used to learn content material in blocks. One software is to show the progress of the obtain.

Within the above instance, the response.physique.getReader() technique returns an iterator. The learn() technique of this traverser returns an object every time, representing the content material block learn this time. The completed attribute of this object is a boolean worth, used to evaluate whether or not it has been learn. The worth attribute is an arrayBuffer array, which represents the content material of the content material block. The worth.size attribute is the dimensions of the present block. content-Switch-Protocol-for-Webpage-%E2percent80percent93-Request-and-Response-Core-JSP-in-Hindi.png

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