所以我想渲染数组，但它一直说，警告：列表中的每个子项都应该有一个唯一的“key”道具，即使它有唯一的键，我的数组包含三个元素，它甚至不能正确渲染第三个数组，由于某种原因，该按钮甚至无法在第三里工作。
import React, { useEffect, useRef } from "react";
import fire from "./firebase";
import firebase from "firebase"
import { useState } from "react"
import Header from "./header"
import Nav from "./nav"
import { NavLink, Redirect } from "react-router-dom";
import AudioPlayer from "./audioplayer"
const Music = ({ match }) => {
const audioRef = useRef();
const audioPlayer = audioRef.current
const [audioSrc, setAudioSrc] = useState()
const [musics, setMusics] = useState([])
const [user, setUser] = useState(null)
const [pfp, setPfp] = useState(null)
const [audioTitle, …我是新来反应的，最近我遇到了这个问题，我不知道如何解决它。它说重新渲染太多。React 限制渲染数量以防止无限循环。无限循环怎么样？是因为 on("value") 吗？
import React from "react";
import fire from "./firebase";
import firebase from "firebase"
import { useState } from "react"
const UserPage = ({ match }) => {
const [user, setUser] = useState(null)
const { params: { userId } } = match;
var userName;
console.log(userId)
firebase.database().ref("users/"+userId+"/praivate/login credentials").on("value", (snapshot)=>{
setUser(snapshot.val().userName)
})
return(
<>
<h1>Hey {user}</h1>
</>
)
}
export default UserPagejavascript firebase reactjs firebase-authentication firebase-realtime-database
我对后端和 Express js 非常陌生。我想从我的其余 api 获取数据，但它发送了此错误 net::ERR_FAILED。
//my api
const express = require("express");
const app = express();
app.get("/", (req, res)=>{
res.send("hello world!!!")
})
const videos = {
"source": "....com",
"url": "...com"
}
app.get("/api/home", (req, res)=>{
res.send(videos)
})
app.listen(3500, ()=>console.log("listening at port 3500..."))<!DOCTYPE html>
<html>
<head>
<title>Hey</title>
</head>
<body>
<button onclick="init()">hey</button>
<script>
function init(){
const url = "http://localhost:3500/api/home"
fetch(url).then(res=>res.json()).then(result=>{
console.log(result)
})
}
</script>
</body>
</html>我想在单击按钮时控制台记录来自 api 的数据videos，但它不起作用。
它甚至说：
从源“http://127.0.0.1:5500”获取“http://localhost:3500/”的访问已被 CORS 策略阻止：请求中不存在“Access-Control-Allow-Origin”标头资源。如果不透明响应满足您的需求，请将请求模式设置为“no-cors”以在禁用 CORS 的情况下获取资源。index.html:12 获取 http://localhost
大家好，我是 React js 的新手，它一直在说 TypeError：无法读取 react.js 中未定义的属性“地图”，即使它不是未定义的。
//my variables
const [musics, setMusics] = useState()
const [user, setUser] = useState(null)
const [pfp, setPfp] = useState(null)
const { params: { uID } } = match;
var userName;
var musicArray = [];
//use effect so it does not infinite loop
useEffect(()=>{ firebase.database().ref("users/"+uID+"/public/songs/").on("value", (snapshot)=>{
var music = snapshot.val()
//gets title of each songs
Object.values(music).forEach((value)=>{
musicArray.push(value.title)//pushes value to array
setMusics(musicArray)//sets that array to musics
})
})
}, [])
return(
.....
<div class = "music-content-container">
<div …