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function idleExec(fn, affectsDOM, timeoutFallbackMs = 500) {
  const root = (typeof window != 'undefined' ? window : global)

  let cancel
  let result
  const thunk = () => { cancel = null; result = fn(); }

  if (affectsDOM && ('requestAnimationFrame' in root)) {
    const handleId = requestAnimationFrame(() => requestAnimationFrame(thunk))
    cancel = () => cancelAnimationFrame(handleId)
  } else if ('requestIdleCallback' in root) {
    const handleId = requestIdleCallback(thunk)
    cancel = () => cancelIdleCallback(handleId)
  } else {
    const handleId = setTimeout(thunk, timeoutFallbackMs)
    cancel = () => clearTimeout(handleId)
  }

  return () => {
    if (cancel !== null) {
      cancel()
      result = fn()
    }
    return result
  }
}

/*
var getFormatter = idleExec(() =>
  new Intl.DateTimeFormat('en-US', {
	timeZone: 'America/Los_Angeles',
  }))

// later in your code, presumably not during the first tick...
var delay = ms => new Promise(res => setTimeout(res, ms))
await delay(1000)
getFormatter().format(new Date(1537452915210)) // => '9/20/2018'
*/