The Fetch API is an interface for fetching resources across the network. It's essentially a replacement for the XMLHttpRequest API, with huge simplifications and using a first class Response and Request object.

Our current implementation for fetchign data from a server in JS is core/ajax. This:

• is written as an AMD module in define module format
• uses jquery to perform all operations
• only has a call function - used to fetch multiple requests in one go
• support synchronous fetches (which block)
• returns jQuery promises

Whilst the XMLHttpRequest object, and jquery, will not be going anywhere any time soon, it would simplify a lot of our interactions to use the Fetch API. Benefits include:

• returning native Promises, which are easier to debug and work with
• supports aborting
• supports keepalive (in Chrome, but not Firefox) as a replacement to the Navigator.sendBeacon API
• a much simpler API
• A much simpler Response object which is easier to work with
• A whole load of options to make it easier to make changs

This issue:

• adds a new core/fetch module with functions:
  • fetchOne() - make a single API call with a simplified API
  • fetchMany() - similar to the core/ajax::call function but with simplified options
• modified core/ajax to call core/fetch in a backwards-compatible fashion
• deprecated the sync option - it should never be used and its use has been strongly discouraged for a very long time
• deprecates the done and fail properties on the requests object passwed to ajax::call because:
  • these are counter-intuitive
  • they tie us to jQuery Promises more tightly
• migrating a bunch of places which use core/ajax.call in central locations to use the new API (and to ESM at the same time)

The core/ajax module is not deprecated as part of this change. I suggest that we consider doing so in the future, but I would recommend that we mark it as deprecated for Moodle 4.5 LTS.