[XHR] Replace XMLHttpRequest with fetch() in Web/API

[wbamberg]

Part of openwebdocs/project#156.

This issue is to track replacement of XHR in code examples with fetch().

I've just searched for XMLHttpRequest and got hits in the following subtrees of Web/API, excluding things which are obviously part of the XHR documentation itself:

• AudioBufferSourceNode
• AudioProcessingEvent
• BaseAudioContext
• Beacon API (false positive)
• ConvolverNode
• Document
• Document Object Model
• DOMParser (false positive)
• EventTarget
• File
• File and Directory Entries API
• File API
• FormData
• FormDataEvent
• History API
• IndexedDB API
• MediaSource
• Navigator (false positive)
• OfflineAudioContext
• PerformanceResourceTiming (false positive)
• ProgressEvent (false positive)
• PushManager
• Request
• Service Worker API (false positive)
• ServiceWorkerRegistration
• SourceBuffer
• Streams API
• Web Audio API
• Web Authentication API (false positive)
• Web Workers API
• WebRTC API
• Worker
• XMLSerializer
• XSLTProcessor

I've deliberately not tried very hard to look for false positives at this point, so there are definitely going to be some in there.

[wbamberg]

I think the example at https://developer.mozilla.org/en-US/docs/Web/API/File_API/Using_files_from_web_applications#handling_the_upload_process_for_a_file should stay XHR, because it wants to show upload progress, and AFAIK, XHR is still the best way to do this (though there is work to support it in fetch() (whatwg/fetch#607). Opinions @teoli2003 ?

[teoli2003]

Yes, it makes sense: Once browsers will provide such a feature with fetch() we will update this entry; meanwhile we should keep it.

Maybe add an explicit sentence saying the Fetch API doesn't allow for this use case?

[wbamberg]

This is done! 🎉