React this.setState未定义

handleClick = () => {
  this.setState({ count: this.state.count + 1 });
}

render() {
  return (
    <button onClick={this.handleClick}>Click me</button>
  );
}

constructor(props) {
  super(props);
  this.handleClick = this.handleClick.bind(this);
}

handleClick() {
  this.setState({ count: this.state.count + 1 });
}

render() {
  return (
    <button onClick={this.handleClick}>Click me</button>
  );
}

handleClick = () => {
  this.setState({ count: this.state.count + 1 });
}

render() {
  return (
    <button onClick={this.handleClick}>Click me</button>
  );
}