Fetch API

For long XMLHttpRequest is being used by web developer’s trusted “amigo”. XMLHttpRequest has enabled ajax and a whole new kind of interactive exposure. 
However, it is being slowly succeeded by the Fetch API. These both deliver the same work i.e. fetching data asynchronously from a different network, but the Fetch API is Promise based. This provides a more cleaner and more concise syntax.
The Fetch API provides the fetch() method defined on a window object. This is used to perform requests. This method returns a Promise which can be further used to retrieve response of the request. 
Basic Syntax:

 fetch(url) //call the fetch function passing the url of the API as a parameter
.then(function(){
  //code for handling data from API
});
.catch(function(){
  //code when the server returns any error
});

NOTE: By default, fetch() will not send or receive any cookies from the server, resulting in unauthenticated requests.
Below are list of methods which can be used when we get a response on what we want to do with the information: 

1. clone(): for creating a clone of response.
2. redirect(): for creating strong response with different url.
3. arrayBuffer(): we return a Promise that resolves with an ArrayBuffer.
4. formData(): also returns a Promise that resolves with a FormDataObject.
5. blob(): same as above only that resolves with a blob.
6. text(): this resolves with a string.
7. json(): it resolves promise with JSON.

Request:
A Request object represents the request part of a fetch call. By passing fetch a Request you can make advanced and customized requests: 

1. method: GET, POST, PUT
2. url: URL of the request
3. headers: Headers object
4. referrer: referrer of the request
5. mode: cors, no-cors, same-origin
6. credentials: should cookies go with the request? omit, same-origin
7. cache: cache mode (default, reload, no-cache)

LOADING JSON
We cannot block the user interface waiting until the request completes. That is why fetch() method returns a Promise. A promise is actually an object which represents a future result. The then() method is used to wait for the response from the server-side and log it to the console.
Below code snippet explains the above logic: 

javascript

fetch('https://www.reddit.com/r/javascript/top/.json?limit=5')
.then(res=>res.json())
.then(json=>console.log(json));

Async…await
This provides a more concise way to process Promises. Its functionality is that we can mark a function as async, then wait for the promise to finish with the await, and access the result as a normal object.
Simple example demonstrating how async…await can be used: 

javascript

async function demo(subject){
   const URL='https://www.reddit.com/r/javascript/top/.json?limit=5';
   const Res= fetch(URL);
   const response= await Res;
   const json= await response.json();
   console.log(json);
}
demo('javascript');

HANDLING ERRORS:
What if we ask the server for a non-existing resource that requires permissions/authorization? With fetch(), we can handle application-level errors like 404 responses.
Although Fetch API is superseding XMLHttpRequest still a beginner needs to learn XMLHttpRequest to make more effective use of Fetch API.
Reference: https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API