Javascript 类型错误:无法读取react js中未定义的属性
我正在尝试使用钩子,但是,我无法找出出现的问题。我想在useEffect中设置令牌值,但我无法定义它Javascript 类型错误:无法读取react js中未定义的属性,javascript,reactjs,Javascript,Reactjs,我正在尝试使用钩子,但是,我无法找出出现的问题。我想在useEffect中设置令牌值,但我无法定义它 import React, { useEffect } from "react"; import "./App.css"; import Login from "./Login"; import { getTokenFromUrl } from "./spotify"; import SpotifyWebApi fr
import React, { useEffect } from "react";
import "./App.css";
import Login from "./Login";
import { getTokenFromUrl } from "./spotify";
import SpotifyWebApi from "spotify-web-api-js";
import Player from "./Player";
import { useDataLayerValue } from "./DataLayer";
const spotify = new SpotifyWebApi();
function App() {
const [{ token }, dispatch] = useDataLayerValue();
useEffect(() => {
const hash = getTokenFromUrl();
window.location.hash = "";
let _token = hash.access_token;
if (_token) {
dispatch({
type: "SET_TOKEN",
token: _token,
});
spotify.setAccessToken(_token);
spotify.getMe().then((user) => {
dispatch({
type: "SET_USER",
user: user,
});
});
}
console.log(token);
}, [dispatch, token]);
return <div className="App">{token ? <Player /> : <Login />}</div>;
}
export default App;
从“React”导入React;
从“react dom”导入react dom;
导入“/index.css”;
从“/App”导入应用程序;
将*作为serviceWorker从“/serviceWorker”导入;
从“/DataLayer”导入{DataLayer};
从“/reducer”导入reducer,{initialState};
ReactDOM.render(
,
document.getElementById(“根”)
);
它引发以下错误:TypeError:无法读取未定义的属性“token”。
我不明白为什么它说“未定义”。任何人都可以帮我做这件事。问题是似乎没有呈现
数据层
,因此数据层上下文
保持未定义
,然后使用数据层价值()
返回未定义
长话短说:您需要将
应用程序
组件包装在数据层中
和/或更改导出const DataLayerContext=createContext()
也许错误来自const[{token},dispatch]=useDataLayerValue()代码>。您正在尝试访问令牌
,但第一个值是未定义的
?是的,错误来自于此。^您应该尝试const val=useDataLayerValue()
,然后打印val
,以查看实际返回的内容,并查看您的解构是否有意义。它返回的是数组吗?是的,它返回的是应用程序组件中的数组,但我仍然无法找到它
import React, { createContext, useContext, useReducer } from "react";
export const DataLayerContext = createContext();
export const DataLayer = ({ reducer, inititalState, children }) => (
<DataLayerContext.Provider value={useReducer(reducer, inititalState)}>
{children}
</DataLayerContext.Provider>
);
export const useDataLayerValue = () => useContext(DataLayerContext);
export const initialState = {
user: null,
playlists: [],
playing: false,
item: null,
token: null,
};
const reducer = (state, action) => {
switch (action.type) {
case "SET_USER":
return {
...state,
user: action.user,
};
case "SET_TOKEN":
return {
...state,
token: action.token,
};
default:
return state;
}
};
export default reducer;
import React from "react";
import ReactDOM from "react-dom";
import "./index.css";
import App from "./App";
import * as serviceWorker from "./serviceWorker";
import { DataLayer } from "./DataLayer";
import reducer, { initialState } from "./reducer";
ReactDOM.render(
<React.StrictMode>
<DataLayer initialState={initialState} reducer={reducer}>
<App />
</DataLayer>
</React.StrictMode>,
document.getElementById("root")
);