如何定义 Ref 的类型

import React, { Component } from 'react'
export default class Demo extends Component {
  formRef: React.RefObject<HTMLElement>;
    render () {
      return (
         <Card bordered={false}>
          <Search
            fields={this.fields}
            WrappedComponentRef={(f) => {
              this.formRef = f;
              return '';
            }}
            handleQuery={this.handleQuery}></Search>
        </Card>
    )
  }
}