Question

我试图通过从<MyContext.Consumer>组件内部调用state上的map来呈现标题列表，但是代码未返回列表项。但是，如果我执行console.log(dataImTryingToMap);，则会准确显示我要呈现的内容。 <ul>被渲染，但是没有<li>。 create-react-app没有引发任何错误。我在想什么？？？这是消费者组件：

<MyContext.Consumer>
    {context => (
        <ul>
            {Object.keys(context.state.subjects).map(subject => {
                <li>{context.state.subjects[subject].title}</li>;
                console.log(context.state.subjects[subject].title);
             })}
        </ul>
    )}
</MyContext.Consumer>

控制台日志完全返回我要查找的数据，但屏幕上没有任何显示。

这是<MyContext.MyProvider>组件的状态：

state = {
    subjects: {
        subject0: {
            title: "Math",
            description: "",
            cards: {
                card1: {
                    note: "",
                    answer: ""
                }
            }
        },
        subject1: {
            title: "history",
            description: "",
            cards: {
                card1: {
                    note: "",
                    answer: ""
                }
            }
        }
    }
};

感谢您的帮助！

Answer 1

您没有从Array.map()回调中返回任何内容：

{Object.keys(context.state.subjects).map(subject => {
    <li>{context.state.subjects[subject].title}</li>; <-- this is not returned
    console.log(context.state.subjects[subject].title);
 })}

const contextValue = { state: {"subjects":{"subject0":{"title":"Math","description":"","cards":{"card1":{"note":"","answer":""}}},"subject1":{"title":"history","description":"","cards":{"card1":{"note":"","answer":""}}}}}};


const MyContext = React.createContext();

const Example = () => (
  <MyContext.Consumer>
  {context => (
    <ul>
    {Object.keys(context.state.subjects).map(subject => {
      console.log(context.state.subjects[subject].title);
      return (
        <li key={subject}>{context.state.subjects[subject].title}</li>
      );
     })}
    </ul>
  )}
  </MyContext.Consumer>
);

const Demo = ({ value }) => (
  <MyContext.Provider value={value}>
    <Example />
  </MyContext.Provider>
);

ReactDOM.render(
  <Demo value={contextValue} />,
  demo
);

<script crossorigin src="https://unpkg.com/react@16.3/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16.3/umd/react-dom.development.js"></script>

<div id="demo">
  </demo>

Answer 2

您还没有从map函数内部返回任何内容，因此它不会呈现任何内容。要么像

那样显式地返回它

var funcs = [];
for (var i = 0; i < 3; i++) {      // let's create 3 functions
  funcs[i] = function(param) {          // and store them in funcs
    console.log("My value: " + param); // each should log its value.
  };
}
for (var j = 0; j < 3; j++) {
  funcs[j](j);                      // and now let's run each one to see with j
}

或隐含喜欢

<MyContext.Consumer>
    {context => (
        <ul>
            {Object.keys(context.state.subjects).map(subject => {
                return <li>{context.state.subjects[subject].title}</li>;
             })}
        </ul>
    )}
</MyContext.Consumer>

从React Context API渲染功能中的地图功能渲染HTML

2 个答案: