loading components twice, probably because of useEffect wrong set-up-CodePudding

I have built a ToDo React App (https://codesandbox.io/s/distracted-easley-zjdrkv) that does the following:

User write down an item in the input bar
User hit "enter"
Item is saved into the list below (local storage, will update later)
There is some logic to parse the text and identify tags (basically if the text goes "@tom:buy milk" --> tag=tom, text=buy milk)

The problem I am facing are:

useEffect runs twice at load, and I don't understand why
After the first item gets saved, if I try saving a second item, the app crashes. Not sure why, but I feel it has to do with the point above...and maybe the event listener "onKeyDown"

App

import { useState, useEffect } from 'react'
import './assets/style.css';
import data from '../data/data.json'


import InputBar from "./components/InputBar/InputBar"
import NavBar from "./components/NavBar/NavBar"
import TabItem from "./components/Tab/TabItem"

function App() {
  const [dataLoaded, setDataLoaded] = useState(
    () => JSON.parse(localStorage.getItem("toDos")) || data
  )

  useEffect(() => {
    localStorage.setItem("toDos", JSON.stringify(dataLoaded))
    console.log('update')
  }, [dataLoaded])



  function deleteItem(id){
    console.log(id)
    setDataLoaded(oldData=>{
      return {
        ...oldData,
        "items":oldData.items.filter(el => el.id !== id)
      }
    })

  }

  
  return (
    <div className='container'>

      <NavBar/>
      <InputBar
        setNewList = {setDataLoaded}
      />
      {
        //Items
        dataLoaded.items.map(el=>{
          console.log(el)
          return  <TabItem item={el} key={el.id} delete={deleteItem}/>
        })
      }   

    </div>
  )
}

export default App

InputBar

import { useState, useEffect } from 'react'
import { nanoid } from 'nanoid'
import '../../assets/style.css';


export default function InputBar(props){

    const timeElapsed = Date.now();
    const today = new Date(timeElapsed);

    function processInput(s) {
       
        let m = s.match(/^(@. ?:)?(. )/)

        if (m) {
            return {
                tags: m[1] ? m[1].slice(1, -1).split('@') : ['default'],
                text: m[2],
                created: today.toDateString(),
                id:nanoid()
            }
        }
    }


    function handleKeyDown(e) {

        console.log(e.target.value)
        console.log(document.querySelector(".main-input-div input").value)

        if(e.keyCode==13){
            props.setNewList(oldData =>{
                return {
                        ...oldData,
                        "items" : [processInput(e.target.value), ...oldData.items]
                    }
                }                
            )
            e.target.value=""
        }
    }

   

   
    return(
        <div className="main-input-div">
            <input type="text" onKeyDown={(e) => handleKeyDown(e)}/>
        </div>
    )
}

Tab

import { useState } from 'react'
import "./tab-item.css"
import { FontAwesomeIcon } from '@fortawesome/react-fontawesome'
import { faTrash } from "@fortawesome/free-solid-svg-icons";



export default function TabItem(props) {


    return (
        <div className="tab-item">
            <div className="tab-item-text">{props.item.text}</div>
            <div className="tab-item-actions">
                <FontAwesomeIcon icon={faTrash} onClick={()=>props.delete(props.item.id)}/>
            </div>
            <div className="tab-item-details">
                <div className="tab-item-details-tags">
                    { 
                        props.item.tags.map(el=><div className="tab-item-details-tags-tag">{el}</div>)
                    }                    
                </div>
            </div>
            <div className="tab-item-date">{props.item.created}</div>
        </div>
    )
}

CodePudding user response：

Your app is using strict mode, which in a development mode renders components twice to help detect bugs (https://reactjs.org/docs/strict-mode.html#detecting-unexpected-side-effects).

root.render(
  <StrictMode>
    <App />
  </StrictMode>
);

As for the crash, I think it's happening due to props.setNewList being an asynchronous call and the resetting of e.target.value - something like this seemed to fix it for me:

function handleKeyDown(e) {

    console.log(e.target.value)
    console.log(document.querySelector(".main-input-div input").value)

    if(e.keyCode==13){
        const inputVal = e.target.value;
        props.setNewList(oldData =>{
            return {
                    ...oldData,
                    "items" : [processInput(inputVal), ...oldData.items]
                }
            }                
        )
        e.target.value=""
    }
}

I will add, that using document.querySelector to get values isn't typical usage of react, and you might want to look into linking the input's value to a react useState hook. https://reactjs.org/docs/forms.html#controlled-components

CodePudding user response：

The above answer is almoost correct. I am adding more info to the same concepts.

useEffect running twice: This is most common ask in recent times. It's because the effect runs twice only in development mode & this behavior is introduced in React 18.0 & above. The objective is to let the developer see & warn of any bugs that may appear due to a lack of cleanup code when a component unmounts. React is basically trying to show you the complete component mounting-unmounting cycle. Note that this behavior is not applicable in the production environment. Please check https://beta-reactjs-org-git-effects-fbopensource.vercel.app/learn/synchronizing-with-effects#step-3-add-cleanup-if-needed for a detailed explanation.

App crashes on second time: It's probably because you are trying to update the input value from event.target.value if you want to have control over the input value, your input should be a controlled component meaning, your react code should handle the onChange of input and store it in a state and pass that state as value to the input element & in your onKeyDown handler, reset the value state. That should fix the crash.

export default function InputBar(props){
    const [inputVal, setInputVal] = useState("");

    function handleKeyDown(e) {
        console.log(e.target.value)
        console.log(document.querySelector(".main-input-div input").value)
        if(e.keyCode==13){
            props.setNewList(oldData =>{
                return {
                        ...oldData,
                        "items" : [processInput(e.target.value), ...oldData.items]
                    }
                }                
            )
            setInputVal("")
        }
    }

    return(
        <div className="main-input-div">
            <input 
            type="text"
            value={inputVal}
            onChange={(e) => {setInputVal(e.target.value)}}
            onKeyDown={(e) => handleKeyDown(e)}
            />
        </div>
    )
}

Hope this helps. Cheers!