React的严格模式StrictMode

StrictMode

严格模式只在开发环境下生效，当执行打包命令也就是生产环境下时，StrictMode会被去除。

React.StrictMode本身就是一个组件，该组件包含的子组件就会在开发环境下开启严格模式。

StrictMode严格模式作用

1. 检测一些危险的操作，比如使用已经废弃的API和不推荐的API
2. 会把生命周期函数都执行两次，来检测额外副作用

// App.js
import { PureComponent } from 'react'
class App extends PureComponent {
  constructor(props) {
    super(props)
    this.state = {
      msg: "hello App"
    }
    console.log("constructor 运行")
  }
  static getDerivedStateFromProps(props, state) {
    console.log(props, state);
    console.log("getDerivedStateFromProps 运行");
    return null;
  }
  render() {
    console.log("render 运行")
    return (
      <div>
        <h1>App组件</h1>
      </div>
    )
  }
  componentDidMount() {
    console.log("componentDidMount 运行")
  }
}

export default App;

// index.js
import React from 'react';
import ReactDOM from 'react-dom/client';
import './index.css';
import App from './App';

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

// If you want to start measuring performance in your app, pass a function
// to log results (for example: reportWebVitals(console.log))
// or send to an analytics endpoint. Learn more: https://bit.ly/CRA-vitals

去掉严格模式：

// index.js
import React from 'react';
import ReactDOM from 'react-dom/client';
import './index.css';
import App from './App';

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
    <App />
);

// If you want to start measuring performance in your app, pass a function
// to log results (for example: reportWebVitals(console.log))
// or send to an analytics endpoint. Learn more: https://bit.ly/CRA-vitals

// App.js
import { PureComponent } from 'react'
class App extends PureComponent {
  constructor(props) {
    super(props)
    this.state = {
      msg: "hello App"
    }
    console.log("constructor 运行")
  }
  static getDerivedStateFromProps(props, state) {
    console.log(props, state);
    console.log("getDerivedStateFromProps 运行");
    return null;
  }
  render() {
    console.log("render 运行")
    return (
      <div>
        <h1>App组件</h1>
      </div>
    )
  }
  componentDidMount() {
    console.log("componentDidMount 运行")
  }
}

export default App;