反应处理 state

Question

I'm trying to make an e-commerce app, and I've just started the project, but I keep getting an error. Every time i try to get the information from my data component and then map through it, I get 'Cannot read property 'map' of undefined'. can someone help me with that? Here are the components:

import React, { useState, useEffect } from 'react';
import './App.css';

import { Routes } from './routes'
import { Main } from './components/Main'
import { storeProducts } from './data/data'

function App() {

  const [items, setItems] = useState([])

  useEffect(() => {
      setItems(storeProducts)
  }, [])

  return (
    <div className="App">
       <Routes />
       <Main items={items} />
    </div>
  );
}

export default App;

import React from 'react'

import { ItemComponent } from './ItemComponent'

export const Main = ({items}) => {

    const products = items.map(() => <ItemComponent />)

    return (
        <div>
           {products}
        </div>
    )
}

import React from 'react'

export const ItemComponent = ({details}) => {
    return (
        <div>
            Hello Item
        </div>
    )
}

data used

export const storeProducts = [
    {
      id: 1,
      title: "Google Pixel - Black",
      img: "img/product-1.png",
      price: 10,
      company: "GOOGLE",
      info:
        "Lorem ipsum dolor amet offal butcher quinoa sustainable gastropub, echo park actually green juice sriracha paleo. Brooklyn sriracha semiotics, DIY coloring book mixtape craft beer sartorial hella blue bottle. Tote bag wolf authentic try-hard put a bird on it mumblecore. Unicorn lumbersexual master cleanse blog hella VHS, vaporware sartorial church-key cardigan single-origin coffee lo-fi organic asymmetrical. Taxidermy semiotics celiac stumptown scenester normcore, ethical helvetica photo booth gentrify.",
      inCart: false,
      count: 0,
      total: 0
    },
    {
      id: 2,
      title: "Samsung S7",
      img: "img/product-2.png",
      price: 16,
      company: "SAMSUNG",
      info:
        "Lorem ipsum dolor amet offal butcher quinoa sustainable gastropub, echo park actually green juice sriracha paleo. Brooklyn sriracha semiotics, DIY coloring book mixtape craft beer sartorial hella blue bottle. Tote bag wolf authentic try-hard put a bird on it mumblecore. Unicorn lumbersexual master cleanse blog hella VHS, vaporware sartorial church-key cardigan single-origin coffee lo-fi organic asymmetrical. Taxidermy semiotics celiac stumptown scenester normcore, ethical helvetica photo booth gentrify.",
      inCart: false,
      count: 0,
      total: 0
    },
    {
      id: 3,
      title: "HTC 10 - Black",
      img: "img/product-3.png",
      price: 8,
      company: "htc",
      info:
        "Lorem ipsum dolor amet offal butcher quinoa sustainable gastropub, echo park actually green juice sriracha paleo. Brooklyn sriracha semiotics, DIY coloring book mixtape craft beer sartorial hella blue bottle. Tote bag wolf authentic try-hard put a bird on it mumblecore. Unicorn lumbersexual master cleanse blog hella VHS, vaporware sartorial church-key cardigan single-origin coffee lo-fi organic asymmetrical. Taxidermy semiotics celiac stumptown scenester normcore, ethical helvetica photo booth gentrify.",
      inCart: false,
      count: 0,
      total: 0
    },
    {
      id: 4,
      title: "HTC 10 - White",
      img: "img/product-4.png",
      price: 18,
      company: "htc",
      info:
        "Lorem ipsum dolor amet offal butcher quinoa sustainable gastropub, echo park actually green juice sriracha paleo. Brooklyn sriracha semiotics, DIY coloring book mixtape craft beer sartorial hella blue bottle. Tote bag wolf authentic try-hard put a bird on it mumblecore. Unicorn lumbersexual master cleanse blog hella VHS, vaporware sartorial church-key cardigan single-origin coffee lo-fi organic asymmetrical. Taxidermy semiotics celiac stumptown scenester normcore, ethical helvetica photo booth gentrify.",
      inCart: false,
      count: 0,
      total: 0
    },
    {
      id: 5,
      title: "HTC Desire 626s",
      img: "img/product-5.png",
      price: 24,
      company: "htc",
      info:
        "Lorem ipsum dolor amet offal butcher quinoa sustainable gastropub, echo park actually green juice sriracha paleo. Brooklyn sriracha semiotics, DIY coloring book mixtape craft beer sartorial hella blue bottle. Tote bag wolf authentic try-hard put a bird on it mumblecore. Unicorn lumbersexual master cleanse blog hella VHS, vaporware sartorial church-key cardigan single-origin coffee lo-fi organic asymmetrical. Taxidermy semiotics celiac stumptown scenester normcore, ethical helvetica photo booth gentrify.",
      inCart: false,
      count: 0,
      total: 0
    },
    {
      id: 6,
      title: "Vintage Iphone",
      img: "img/product-6.png",
      price: 17,
      company: "apple",
      info:
        "Lorem ipsum dolor amet offal butcher quinoa sustainable gastropub, echo park actually green juice sriracha paleo. Brooklyn sriracha semiotics, DIY coloring book mixtape craft beer sartorial hella blue bottle. Tote bag wolf authentic try-hard put a bird on it mumblecore. Unicorn lumbersexual master cleanse blog hella VHS, vaporware sartorial church-key cardigan single-origin coffee lo-fi organic asymmetrical. Taxidermy semiotics celiac stumptown scenester normcore, ethical helvetica photo booth gentrify.",
      inCart: false,
      count: 0,
      total: 0
    },
    {
      id: 7,
      title: "Iphone 7",
      img: "img/product-7.png",
      price: 30,
      company: "apple",
      info:
        "Lorem ipsum dolor amet offal butcher quinoa sustainable gastropub, echo park actually green juice sriracha paleo. Brooklyn sriracha semiotics, DIY coloring book mixtape craft beer sartorial hella blue bottle. Tote bag wolf authentic try-hard put a bird on it mumblecore. Unicorn lumbersexual master cleanse blog hella VHS, vaporware sartorial church-key cardigan single-origin coffee lo-fi organic asymmetrical. Taxidermy semiotics celiac stumptown scenester normcore, ethical helvetica photo booth gentrify.",
      inCart: false,
      count: 0,
      total: 0
    },
    {
      id: 8,
      title: "Smashed Iphone",
      img: "img/product-8.png",
      price: 2,
      company: "apple",
      info:
        "Lorem ipsum dolor amet offal butcher quinoa sustainable gastropub, echo park actually green juice sriracha paleo. Brooklyn sriracha semiotics, DIY coloring book mixtape craft beer sartorial hella blue bottle. Tote bag wolf authentic try-hard put a bird on it mumblecore. Unicorn lumbersexual master cleanse blog hella VHS, vaporware sartorial church-key cardigan single-origin coffee lo-fi organic asymmetrical. Taxidermy semiotics celiac stumptown scenester normcore, ethical helvetica photo booth gentrify.",
      inCart: false,
      count: 0,
      total: 0
    }
  ];

Answer 1

Seems strange but try this:

export const Main = ({items}) => {

    if (!items) return <div/>

    const products = items.map(() => <ItemComponent />)

    return (
        <div>
           {products}
        </div>
    )
}

And see if it renders. setState is async and sometimes that causes this issue.

Answer 2

Your error mean that you use .map function on a variable which has undefined value. Here I have some feedbacks to improve your code:

No need to use useEffect in App component:

function App() {   
  const [items, setItems] = useState(storeProducts);

  return (
    <div className="App">
       <Routes />
       <Main items={items} />
    </div>
  );
}

And Main component:

export const Main = ({items}) => {
    const products = (items || []).map(() => <ItemComponent />)

    return (
        <div>
           {products}
        </div>
    )
}

items || [] mean: if items is undefined then use empty array. And then no error "Cannot read property 'map' of undefined" anymore.