React 16.13.1开启严格模式会触发两次render

看代码

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

　　这时候组件里的render()会执行两次

import React from 'react';
export default class About extends React.Component {
  render() {
    console.log('---about-');
    return (
      <div>
        Here is About!
        <div>
          传过来的值：{this.props.match.params.id}
          <br/>
          中文：{this.props.match.params.text}
        </div>
      </div>
    )
  }
}

　　

strict mode的通过两次调用constructor和render函数来更好的检测不符合预期的side effects

文档中有表明

Strict mode can’t automatically detect side effects for you, but it can help you spot them by making them a little more deterministic. This is done by intentionally double-invoking the following functions:

• Class component constructor, render, and shouldComponentUpdate methods
• Class component static getDerivedStateFromProps method
• Function component bodies
• State updater functions (the first argument to setState)
• Functions passed to useState, useMemo, or useReducer

下列函数会执行两次

• 类组件的constructor,render和shouldComponentUpdate方法
• 类组建的静态方法getDerivedStateFromProps
• 函数组件方法体
• 状态更新函数(setState的第一个参数)
• 传入useState,useMemo或useReducer的函数

在production环境下不会这样,所以不用担心