React seems to re-render the wrong component when triggering another one-CodePudding

I have two components in my project.

One is Aapp.jsx

One is Child.jsx

When I called the state set function in Child 1; it is supposed to see mentioning child 1 in the console, but now it is showing child 3. It is weird and why is that?

I guess the reason is "key" in prop, so I added "key" there too. But the problem is still there.

Here is the code: App:

import React,{useState,useEffect} from 'react';
import {Child} from './Child.jsx'
export function App(props) {

  [message,setMessage]=useState('');
  [showChild1,setShowChild1]=useState(true);
  [showChild2,setShowChild2]=useState(true);
  [showChild3,setShowChild3]=useState(true);

[child1data,setChild1data] = useState('child1');
[child2data,setChild2data] = useState('child2');
[child3data,setChild3data] = useState('child3');

  useEffect(() => {
    console.log('parent was rendered')
  })
  return (
    <div className='App'>
      

<button onClick={()=>setShowChild1(!showChild1)}>Show child1</button>
{showChild1 && <Child key='1' data={child1data}/>}
<br/>
<br/>
<button onClick={()=>setShowChild2(!showChild2)}>Show child2</button>
{showChild2 && <Child key='2'data={child2data}/>}

<br/>
<br/>
<button onClick={()=>setShowChild3(!showChild3)}>Show child3</button>
<br/>
{showChild3 && <Child key='3' data={child3data}/>}
      

    </div>
  );
}

// Log to console
console.log('Hello console')

Child:

import React, {useState, useEffect} from 'react';

export const Child = (props) => {

const {data} = props;

[message,setMessage]=useState('');

useEffect(()=>{
  console.log(data)
  console.log(message)
})

  return <>
  
  <h1>This is {data}</h1>
  <input onChange={((e)=>setMessage(e.target.value))}></input>
  
  </>
}

For better illustrate, here is my code https://playcode.io/940717

CodePudding user response：

This is a tricky one but fortunately comes with a very simple fix.

TL;DR

In Child.jsx change this:

  [message, setMessage] = useState('');

to this:

  const [message, setMessage] = useState('');

The longer answer

When declaring variables without let, const, or var (in a non-strict environment) Javascript will create an implicit global variable. What this means is that your message and setMessage variables point to the last value they were assigned. I.e. The global variable message will be assigned to the result of the useState call in the last Child component that you render, which in your case is Child 3.

So, when you modify message, Child 3 detects that change and runs the effect for Child 3, not the Child component where the change was actually made.

You can see this in action by changing the Child component to this and examining the output in the console:

  [message, setMessage] = useState("");

  useEffect(() => {
    console.log(data); // Will always log "child 3"
    console.log(message);
  });

  return (
    <>
      <h1>This is {data}</h1>
      <input
        onChange={(e) => {
          console.log(data); // Will log "child n" where n is the child you expect
          setMessage(e.target.value);
        }}
      ></input>
    </>
  );

You might also be curious as to why, if all of your Child components are referencing the same message variable, you're still able to change the inputs individually. This is because you're input elements are uncontrolled and their state is being managed by the browser natively.

You can see this in action by adding the following to the input element in your Child component:

  <input value={message} onChange={e => setMessage(e.target.value)}></input>

What to do in the future

You should always, always, always declare your variables with let or const as it will save you headaches like this in the future. In fact, many modern transpilers such as Babel will throw an error if you don't.

When using uncontrolled inputs you should consider asking yourself why you need to do so. More often than not, a controlled input is what you actually want and in the cases where it's not, consider leaving a comment as to why it's not.

CodePudding user response：

You forgot const in many places:

const [message, setMessage] = useState('');

Without the const keyword message and setMessage (and other stuff) leak to the window global object.

Working example