XMLHttpRequest, reimplemented using fetch.

• This allows code using XHR to be tested using Node, with our friend node-fetch.
• You can also use it inside service workers!

Due to the nature of fetch(),

• This does not support synchronous XHR.
• This does not support progress event.
• This does not implement XMLHttpRequest.upload.
• XMLHttpRequest instance is not reusable.
• HEADERS_RECEIVED and LOADING ready states are skipped.

Because this is still XMLHttpRequest,

• This does not support omitting cookies for same-origin requests.
• There is no Promise support.
• There is no built-in Node.js support. Polyfill using node-fetch.

This does not support obsolete APIs such as:

• XMLHttpRequest.onreadystatechange
• Basic authentication in XMLHttpRequest.open (use Authorization header!)

Due to the nature of the main usage,

• XMLHttpRequest.responseXML always throws.
• Document cannot be used in XMLHttpRequest.send().
• The default MIME type is text/plain.
• Instead of ProgressEvent, CustomEvent is used.

This shim assumes you have these ES features:

• async support
• class syntax
• globalThis
• Object.assign
• Object.create
• Object.entries
• Promise including Promise.prototype.finally
• Symbol
• Template literals

And these WHATWG features:

• AbortController
• Blob
• Constructible DOMException
• EventTarget.prototype.addEventListener with once option
• fetch API with signal support
• setTimeout