setState到底是同步还是异步？

import React, { Component } from "react";

export default class AsyncState extends Component {
  state = {
    count: 0,
  };

  //React 有个概念，叫做batchUpdate(批量更新)
  handleClick = () => {
    //batchedUpdating = true（开始）
    //18:这部分是异步的 17：也是异步的
    this.setState({ count: 1 });
    console.log("this.state.count:", this.state.count);
    this.setState({ count: 2 });
    console.log("this.state.count:", this.state.count);
    //setTimeout不参与batchedUpdating过程
    setTimeout(() => {
      //18:这部分是异步的 17：同步的
      this.setState({ count: 3 });
      console.log("this.state.count:", this.state.count); //2,3
      this.setState({ count: 4 });
      console.log("this.state.count:", this.state.count); //2,4
    }, 0);
    //batchedUpdating = false（结束）
  };
  render() {
    return (
      <div>
        <h5>{this.state.count}</h5>
        <button onClick={this.handleClick}>click</button>
      </div>
    );
  }
}