Simply try the below code and you may understand
import React, { Component } from 'react';
class App extends Component {
constructor(props) {
super(props);
}
showFile = async (e) => {
e.preventDefault()
const reader = new FileReader()
reader.onload = async (e) => {
const text = (e.target.result)
console.log(text)
alert(text)
};
reader.readAsText(e.target.files[0])
}
render = () => {
return (<div>
<input type="file" onChange={(e) => this.showFile(e)} />
</div>
)
}
}
export default App;