detect global JS feature

detect.js

// from https://github.com/github/fetch/blob/master/fetch.js
// detecting the existince of fetch() and only patching if it is absent


var global =
  (typeof globalThis !== 'undefined' && globalThis) ||
  (typeof self !== 'undefined' && self) ||
  (typeof global !== 'undefined' && global) ||
  {}

export function fetch(input, init) {
  return new Promise(function(resolve, reject) {
   // ...
  }
}
                     
fetch.polyfill = true

if (!global.fetch) {
  global.fetch = fetch
  global.Headers = Headers
  global.Request = Request
  global.Response = Response
}