写一个方法实现指定开始的数字增加到指定的结束数字，步长默认为1

Answer 1

/**
*

• @param Number start
• @param Number end
• @param Number step default = 1
• @param Number delay (ms) delay to exec
• @param Function cb callback, have two arguments: currentNumber, end
  */
  function addTo(start, end, step = 1, delay, cb) {
  if (start >= end ) {
  start = end
  cb && cb(start, end)
  return
  }
  return new Promise((resolve, reject) => {
  if (delay) {
  setTimeout(() => {
  cb && cb(start, end)
  start = start += step
  addTo(start, end, step, delay, cb)
  }, delay);
  } else {
  cb && cb(start, end)
  start = start += step
  addTo(start, end, step, delay, cb)
  }
  })
  }
  // test
  addTo(2, 100, 4, null, console.log)