react 异步的setState

import React from "react";

class App extends React.Component {
  state = {
    a: 0
  };
  componentDidMount() {
    this.setState({ a: this.state.a + 1 });
    console.log(this.state.a); // 0

    this.setState({ a: this.state.a + 1 });
    console.log(this.state.a); // 0

    setTimeout(() => {
      this.setState({ a: this.state.a + 1 });
      console.log(this.state.a); // 2

      this.setState({ a: this.state.a + 1 });
      console.log(this.state.a); // 3
    }, 0);
  }

  render() {
    return <>hello world</>;
  }
}

export default App;