JSFiddle - React, Tailwind, and code Playground

HTML

<div id="temp">
    <div id="bg-prev" ref="prev">
        <h1>{{x}}</h1>
    </div>
<div>

CSS

#bg-prev {
  width: 100%;
}

JavaScript

new Vue({
	el: '#temp',
  data: {
  	x: 0
  },
  mounted() {
  	this.getWindowWidth()
  },
  methods: {
    getWindowWidth() {
      this.x = this.$refs.prev.clientWidth;
    }
  }
})