React 16.13.1使用useState时会执行两次render的问题

示例使用typescript编写一个简单的hello

新建一个Hello.tsx组件：

import React, {useState} from 'react'


interface IHelloProps {
    message?: string;
}
const Hello: React.FC<IHelloProps> = (props) => {
    console.log('hello before render....')
    return (
        <h2>
            {props.message}
        </h2>
    )
}

export default Hello

Hello.defaultProps = {
    message: 'what the fuck'
}

观察控制台输出，会发现，hello before render....只输出一次。

继续往hello组件中添加count状态

import React, {useState} from 'react'


interface IHelloProps {
    message?: string;
}
const Hello: React.FC<IHelloProps> = (props) => {
    const [count, setCount] = useState(0)

    console.log('hello before render....')
    return (
        <h2>
            {props.message}
            <div>点击次数{count}</div>
        </h2>
    )
}

export default Hello

Hello.defaultProps = {
    message: 'what the fuck'
}

观察控制台输出，会发现hello before render....输出了两次，

继续往hello添加点击事件：

import React, {useState} from 'react'


interface IHelloProps {
    message?: string;
}
const Hello: React.FC<IHelloProps> = (props) => {
    const [count, setCount] = useState(0)

    console.log('hello before render....')
    return (
        <h2>
            {props.message}
            <div
                onClick={() => {
                    setCount(count + 1)
                }}
            >
                点击次数{count}
            </div>
        </h2>
    )
}

export default Hello

Hello.defaultProps = {
    message: 'what the fuck'
}

观察控制台输出，会发现初始化的时候，hello before render....执行了两次，每次点击的时候，hello before render....都输出了两次。

这个问题产生的原因是啥，可以从https://reactjs.org/docs/strict-mode.html#detecting-unexpected-side-effects找到答案

原因:

最近的react版本,dev模式下render使用的是strict mode,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