async-idemp-wait.js

/**
 * Runs the `idempotentFn` for ~4400ms, in an interval
 * of 50ms - to satisfy `testFn`. If unsatisfied, throws Error(`msg`).
 * 
 * @param idempotentFn - a function that can be executed ∞ times
 * without a side-effect.
 * @param testFn - throw if unsatisfied; if no throw, then return perfectly ✓!
 * @param msg - error message to throw if no luck at all
 */
async function waitAndSee (idempotentFn, testFn, msg) {
  let countMs = 0
  const stepMs = 50
  const totalMs = 4400 - stepMs
  const sleep = ms => new Promise(r => setTimeout(r, ms))

  function log (str, lineEnd = '\n', tab = ' '.repeat(6)) {
    if (typeof str === 'object') str = util.inspect(str)
    process.stdout.write(tab + (str) + lineEnd)
  }

  const logEnd = ms => log('] ' + ms + 'ms', '\n', '')

  log('[', '')
  while (countMs < totalMs) {
    const start = new Date()
    const retval = await idempotentFn()
    let okay = false

    try {
      // @ts-ignore
      await testFn(retval)
      okay = true
    } catch (err) {
      okay = false
    }

    const ms = new Date().getTime() - start.getTime()
    countMs += ms

    if (okay) {
      logEnd(countMs)
      return retval
    }

    log('-', '', '')
    countMs += stepMs
    await sleep(stepMs)
  }
  logEnd(countMs)
  throw Error(msg)
}