如何在不重置表单的情况下使用ajax和codeigniter上传图片？

<form id="uploadForm" enctype="multipart/form-data">
  <input type="file" name="image" id="imageInput">
  <button type="button" onclick="uploadImage()">上传图片</button>
</form>

<script>
  function uploadImage() {
    var form = document.getElementById('uploadForm');
    var formData = new FormData(form);

    var xhr = new XMLHttpRequest();
    xhr.open('POST', '/upload/image', true);
    xhr.onload = function() {
      if (xhr.status === 200) {
        console.log('图片上传成功');
      } else {
        console.log('图片上传失败');
      }
    };
    xhr.send(formData);
  }
</script>

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Upload extends CI_Controller {
  public function image() {
    $config['upload_path'] = './uploads/';
    $config['allowed_types'] = 'gif|jpg|png';
    $config['max_size'] = 2048;

    $this->load->library('upload', $config);

    if (!$this->upload->do_upload('image')) {
      echo '图片上传失败';
    } else {
      $data = $this->upload->data();
      echo '图片上传成功，保存路径：' . $data['full_path'];
    }
  }
}