SOURCE

window.onload = function () {
  let e = document.querySelector('textarea.input')
  // e.addEventListener('input', function () {
  //     this.style.height='auto'
  //     this.style.height = this.scrollHeight + 'px'
  // })
  autoTextarea(e)
}

function autoTextarea (elem, extra, maxHeight) {
  extra = extra || 0
  let isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window
  let isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera')
  let addEvent = function (type, callback) {
    elem.addEventListener ? elem.addEventListener(type, callback, false) : elem.attachEvent('on' + type, callback)
  }
  let getStyle = elem.currentStyle ? function (name) {
    var val = elem.currentStyle[name]
    if (name === 'height' && val.search(/px/i) !== 1) {
      var rect = elem.getBoundingClientRect()
      return rect.bottom - rect.top -
        parseFloat(getStyle('paddingTop')) -
        parseFloat(getStyle('paddingBottom')) + 'px'
    }
    return val
  } : function (name) {
    return getComputedStyle(elem, null)[name]
  }
  let minHeight = parseFloat(getStyle('height'))

  elem.style.resize = 'none'

  let change = function () {
    let scrollTop
    let height
    let padding = 0
    let style = elem.style

    if (elem._length === elem.value.length) return
    elem._length = elem.value.length

    if (!isFirefox && !isOpera) {
      padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'))
    }
    scrollTop = document.body.scrollTop || document.documentElement.scrollTop

    elem.style.height = minHeight + 'px'
    if (elem.scrollHeight > minHeight) {
      if (maxHeight && elem.scrollHeight > maxHeight) {
        height = maxHeight - padding
        style.overflowY = 'auto'
      } else {
        height = elem.scrollHeight - padding
        style.overflowY = 'hidden'
      }
      style.height = height + extra + 'px'
      scrollTop += parseInt(style.height) - elem.currHeight
      document.body.scrollTop = scrollTop
      document.documentElement.scrollTop = scrollTop
      elem.currHeight = parseInt(style.height)
    }
  }
  addEvent('propertychange', change)
  addEvent('input', change)
  addEvent('focus', change)
  change()
}

<textarea class="input" placeholder="textarea高度自适应" rows="3" cols="50">
</textarea>

textarea.input {
  line-height: 24px;
  outline: none;
}