node_modules/@supabase/functions-js/src/helper.ts

import { Fetch } from './types'

export const resolveFetch = (customFetch?: Fetch): Fetch => {
  let _fetch: Fetch
  if (customFetch) {
    _fetch = customFetch
  } else if (typeof fetch === 'undefined') {
    _fetch = (...args) =>
      import('@supabase/node-fetch' as any).then(({ default: fetch }) => fetch(...args))
  } else {
    _fetch = fetch
  }
  return (...args) => _fetch(...args)
}