React 组件渲染两次（useEffect 运行两次）

Question

Hi i'm new to react js and i was wondering why my component is rendering twice(useeffect runs twice). I tried removing strict mode and it renders only once but even then i don't understand why count is shown as 01 instead of just only 1. if i add the event handler the component breaks and i get that newarray[id][1]-1 is undefined (the function runs even if i don't click - or +)

import React, { useEffect, useState } from "react";
import { useOutletContext } from "react-router-dom";




function ShoppingCart(prop) {

  const [productstobuy, setProducts] = useState([]);
  const [count, setCount] = useState([0])
  console.log(productstobuy)
/*
   const handlePlusMinus = (sign,id) => {
    console.log("i'm in")
     const newarray = [...productstobuy]
     console.log(newarray)
     if (sign === "+") {
       newarray[id][1] = newarray[id][1]+1
     } else if ((newarray[id][1]-1) >= 1) {
      newarray[id][1] = newarray[id][1]+1
     }
     setProducts(newarray);
     
    }
 */



  const allItems = useOutletContext()

  useEffect(() => {
    setCount(count + 1)
    console.log("called")
    let toadd = allItems.filter((elem, index) => { return index === prop.index })
    console.log(toadd);
    let productexsist = productstobuy.filter((elem) => { return elem[0] === toadd })
    console.log(productexsist.length)


    if (!productexsist.length) {
      console.log([(allItems.filter((elem, index) => { return index === prop.index }))[0], 1])
      setProducts(oldArray => [...oldArray, [toadd[0], 1]]);
      console.log(productstobuy)

    } else {
      console.log("wrong")
    }

  }, [prop.index])







  return (
    <div>
      {count}
      {console.log(productstobuy)}
      {productstobuy.length > 0 ? productstobuy.map((product) => {
        console.log(product[1])
        return (<div className="product" id={prop.index}>{product[0]}<div><button id={prop.index} /*onClick={handlePlusMinus("-",prop.index)}*/>-</button><input type="number" value={product[1]} /><button id={prop.index} /*onClick={handlePlusMinus("-",prop.index)}*/>+</button></div></div>)


      }) : null}
    </div>




  );




}


export { ShoppingCart }

Answer 1

why count is shown as 01 instead of just only 1

It's because of this line const [count, setCount] = useState([0]) . count should be initialised as a number instead of an array. What actually happened in the useEffect was setCount([0]+1) instead of the intended setCount(0+1)

the function runs even if i don't click - or +

Instead of putting onClick={handlePlusMinus("-",prop.index)} , try onClick={() => handlePlusMinus("-",prop.index)} instead. this would ensure that the handlePlusMinus function is only called when the element is clicked. Elaboration here: React onClick function fires on render

Answer 2

It renders twice because of the Strict Mode.
https://reactjs.org/docs/strict-mode.html