input 上传文件转bsae64

<input type="file" id="choose" accept="application/pdf">

　　

var file = document.getElementById('choose')
file.onchange = function () {
  var reader = new FileReader()
  reader.onload = function (ev) {
    var base64 = ev.target.result  
  }        
  reader.readAsDataURL(file)
}