Map function 在 React JSX 中不返回項目

Question

在我的 PlaylistsLinks 組件之外，我初始化folders變量，然后運行 Firebase function 迭代所有文件夾名稱並將其推送到folders數組。 我的 map function 不會遍歷我的folders數組，即使我可以在我的組件中 console.log 我的數組。 為了使用map ，我必須以特定的方式將數組傳遞到功能組件中嗎？

import React, { useState, useEffect, useRef } from 'react';
import firebase from "firebase/app";
import storage from "firebase/storage";
import  firebaseConfig from "../dropzone/config";



if (!firebase.apps.length) {
    firebase.initializeApp(firebaseConfig);

 }else {
    firebase.app(); // if already initialized, use that one
 }

let fb_storage = firebase.storage();
let storageRef = fb_storage.ref();  

let rootRef = storageRef.root;

let folders = [];



rootRef.listAll().then(function(res) {
    res.prefixes.forEach(function(folderRef) {
        // console.log(folderRef.name)
        folders.push(folderRef.name)
        
        // All the prefixes under listRef.
        // You may call listAll() recursively on them.
    });
    res.items.forEach(function(itemRef) {
        // All the items under listRef.
        // console.log(itemRef.name);
    });
    }).catch(function(error) {
    // Uh-oh, an error occurred!
    }); 

   

const PlayListLinks = () =>{
    
   
console.log(folders);

    return(
        <>
            <ul>
                {folders.map((folder, index) => {
                    return <li key={index}> {folder}</li>
                })}
            </ul>
        </>
    )
}

export default PlayListLinks

Answer 1

創建一個 state 然后用獲取的文件夾列表更新它：

import React, { useState, useEffect, useRef } from 'react';
import firebase from "firebase/app";
import storage from "firebase/storage";
import  firebaseConfig from "../dropzone/config";

if (!firebase.apps.length) {
    firebase.initializeApp(firebaseConfig);

 }else {
    firebase.app(); // if already initialized, use that one
 }

let fb_storage = firebase.storage();
let storageRef = fb_storage.ref();  

let rootRef = storageRef.root;

const PlayListLinks = () =>{
const [folders, setFolders] = useState([])

useEffect(()=>{
    rootRef.listAll().then(function(res) {
    let temp = []
    res.prefixes.forEach(function(folderRef) {
        temp.push(folderRef.name)
    });
    setFolders(temp);
    res.items.forEach(function(itemRef) {
        // All the items under listRef.
        // console.log(itemRef.name);
    });
    }).catch(function(error) {
    // Uh-oh, an error occurred!
    }); 

},[])
   
console.log(folders);

    return(
        <>
            <ul>
                {folders.map((folder, index) => {
                    return <li key={index}> {folder}</li>
                })}
            </ul>
        </>
    )
}

export default PlayListLinks

Answer 2

問題是您必須直接從 function 內返回您的 JSX。map

像這樣：

{folders.map((folder, index) => <li key={index}>{folder}</li>)}

或者：

{folders.map((folder, index) => (
    <li key={index}>{folder}</li>
)}