Fetch is browser API ,it is used to make AJAX call to server without disturbing full page of application, it resides in window object of the browser.
Fetch API send a AJAX request with corresponding input request[User provided ] to server then server response to that request.
.then(response => response.json())
.then(json => console.log(json)).catch(err=>console.log(err))
Headers :- provide header information like content-type, Auth -token etc..
Method :- Type of http request like GET , POST ,PUT , DELETE, PATCH etc…
Then and catch :- Both are callbacks, Then executes once request resolves successfully, it may contains multiple then block first then block response is passed to another then block