let watermark = {}
 
let setWatermark = (text, sourceBody) => {
  let id = Math.random()*10000+'-'+Math.random()*10000+'/'+Math.random()*10000
 
  if (document.getElementById(id) !== null) {
    document.body.removeChild(document.getElementById(id))
  }
 
  let can = document.createElement('canvas')
  can.width = 150
  can.height = 120
 
  let cans = can.getContext('2d')
  cans.rotate(-20 * Math.PI / 180)
  cans.font = '15px Vedana'
  cans.fillStyle = 'rgba(0, 0, 0, .5)'
  cans.textAlign = 'left'
  cans.textBaseline = 'Middle'
  cans.fillText(text, can.width / 20, can.height )
 
  let water_div = document.createElement('div')
  water_div.id = id
  water_div.style.pointerevents = 'none'
  water_div.style.background = 'url(' + can.toDataURL('image/png') + ') left top rePEat'
  if(sourceBody){
    water_div.style.width = '100%'
    water_div.style.height = '100%'
    sourceBody.appendChild(water_div)
  }else{
    water_div.style.top = '3px'
    water_div.style.left = '0px'
    water_div.style.posITion = 'fixed'
    water_div.style.zIndex = '100000'
    water_div.style.width = document.documentElement.clientWidth  + 'px'
    water_div.style.height = document.documentElement.clientHeight  + 'px'
    document.body.appendChild(water_div)
  }
 
  return id
}
 
/**
 *  该方法只允许调用一次
 *  @param:
 *  @text == 水印内容
 *  @sourceBody == 水印添加在哪里，不传就是body
 * */
watermark.set = (text, sourceBody) => {
  let id = setWatermark(text, sourceBody)
  setInterval(() => {
    if (document.getElementById(id) === null) {
      id = setWatermark(text, sourceBody)
    }
  }, 2000)
  window.onresize = () => {
    setWatermark(text, sourceBody)
  }
}
 
export default watermark

// 水印
import watermark From './utils/watermark.js'
Vue.PRototype.$watermark = watermark

this.$watermark.set("浩星2731")

<el-button @click="addWatermark">点我添加水印</el-button>
<div ref="content" style="width: 500px;height: 500px;border: 1px solid #ccc;">
 addWatermark(){
      this.$watermark.set("浩星2731",this.$refs.content)
    }

can.width = 150
can.height = 120