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 info contained in Response is learn asynchronously by the Stream interface, nevertheless 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:

  • fundamental: Abnormal, 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 ready to no-cors, this response worth can be returned.
  • opaqueredirect: If the redirect attribute of the fetch() request is ready to handbook, this response worth can 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 vital level to notice: fetch() will report an error solely when there’s a community error or can’t join. In different circumstances, no error can 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 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 have to be equal to 2xx (200~299) to find out that the request is profitable. There’s no want to contemplate the URL soar (standing code is 3xx) as a result of fetch() will routinely convert the jumped standing code to 200. One other methodology 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 offers the next strategies to govern headers.

  • Headers.get(): In response to the desired key identify, return the key-value.
  • Headers.has(): Returns a Boolean worth indicating whether or not a header is included.
  • Headers.set(): Set the desired key identify as the brand new key-value, if the important thing identify doesn’t exist, it will likely 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 perform.

A number 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 permit 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() methodology also can traverse all key values ​​and key names.

The best way to learn content material

The Response object offers completely different studying strategies based on several types of knowledge 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 will need to wait till the tip of the asynchronous operation to get the whole knowledge returned by the server.

response.textual content()

response.textual content() can be utilized to get textual content knowledge, resembling HTML information.


response.json() is principally used to get the JSON knowledge 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 consumer, modify some knowledge, 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 net 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 music.ogg after which performs it on-line.


The Stream object can solely be learn as soon as and it’s gone after studying. Because of this solely one of many 5 studying strategies within the earlier part can be utilized, in any other case, an error can 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 offers the response.clone() methodology, 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() methodology, which is used to redirect the Response consequence to the desired URL. This methodology is usually 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 consumer operations. It may be used to learn content material in blocks. One utility is to show the progress of the obtain.

Within the above instance, the response.physique.getReader() methodology returns an iterator. The learn() methodology of this traverser returns an object every time, representing the content material block learn this time. The carried out 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 scale of the present block. content-Switch-Protocol-for-Webpage-%E2percent80percent93-Request-and-Response-Core-JSP-in-Hindi.png

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