JS FileReader How to get the contents of the.txt file

<input type="file" id="file-selector" accept=".txt" ref="fileSelector">

const fileSelector = document.getElementById('file-selector');
    fileSelector.addEventListener('change', ()=>{
      const reader = new FileReader()
      reader.readAsText(fileSelector.files[0],'utf8') // input.files[0]为第一个文件
      reader.onload = ()=>{
        document.body.innerHTML += reader.result  // reader.result为获取结果
      }
    }, false)

in Vue,I want Read the txt file in JavaScript,what is wrong with above code