this.state不支持react js中的reader.onload函数

import React from 'react';

class FileUploadComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      fileContent: null,
    };
  }

  handleFileUpload = (event) => {
    const file = event.target.files[0];
    const reader = new FileReader();

    reader.onload = () => {
      const content = reader.result;
      this.setState({ fileContent: content });
    };

    reader.readAsText(file);
  }

  render() {
    return (
      <div>
        <input type="file" onChange={this.handleFileUpload} />
        <p>File content: {this.state.fileContent}</p>
      </div>
    );
  }
}

export default FileUploadComponent;