函数防抖和节流

function debounce(func, wait, immediate = true) {
  let timer = 0

  return function(...args) {
    if (timer) {
      clearTimeout(timer)
    } else {
      if (immediate) {
        func.apply(func, args)
      }
    }

    timer = setTimeout(() => {
      timer = 0
      if (!immediate) {
        func.apply(func, args)
      }
    }, wait)
  }
}

function getNow() {
  return + new Date()
}


function throttle(func, wait) {
  let [now, previous, timer, remainTime] = [0, 0, 0, 0]

  const later = (...params) => setTimeout(() => {
    func.apply(func, params)
    previous = now
    timer = 0
  }, remainTime)

  return function(...params) {
    now = getNow()
    // if if's the first time
    if (!previous) {
      previous = now
    }

    remainTime = wait - (now - previous)
    // if there is not a timer, then create it
    if (!timer) {
      timer = later(...params)
    }
  }
}