file-reader.js

import * as React from 'react'
import {useState} from 'react'
import ReactDOM from 'react-dom'

function FileReader() {
  const [text, setText] = useState('')

  function onChange(e: React.ChangeEvent<HTMLInputElement>) {
    const {files} = e.target
    const [file] = files

    console.log('File:', file)

    const reader = new window.FileReader()
    reader.onload = e => setText(String(e.target.result))

    reader.readAsText(file)
  }

  return (
    <div>
      <div>
        <input type="file" onChange={onChange} />
      </div>

      <code>{text}</code>
    </div>
  )
}

ReactDOM.render(<FileReader />, document.querySelector('#app'))