React的列表组件必须要有key？

function ListItem(props) {
    const value = props.value;
    return (
        // 错误！你不需要在这里指定 key：
        <li key={value.toString()}>
            {value}
        </li>
    );
}

function NumberList(props) {
    const numbers = props.numbers;
    const listItems = numbers.map((number) =>
        // 错误！元素的 key 应该在这里指定：
        <ListItem value={number} />
    );
    return (
        <ul>
            {listItems}
        </ul>
    );
}

const numbers = [1, 2, 3, 4, 5];
ReactDOM.render(
    <NumberList numbers={numbers} />,
    document.getElementById('root')
);

function ListItem(props) {
    // 正确！这里不需要指定 key：
    return <li>{props.value}</li>;
}

function NumberList(props) {
    const numbers = props.numbers;
    const listItems = numbers.map((number) =>
        // 正确！key 应该在数组的上下文中被指定
        <ListItem key={number.toString()} value={number} />
    );
    return (
        <ul>
            {listItems}
        </ul>
    );
}

const numbers = [1, 2, 3, 4, 5];
ReactDOM.render(
    <NumberList numbers={numbers} />,
    document.getElementById('root')
);

# Post 组件可以读出 props.id，但是不能读出 props.key
const content = posts.map((post) =>
    <Post
        key={post.id}
        id={post.id}
        title={post.title} />
);

function Blog(props) {
    const sidebar = (
        <ul>
            {props.posts.map((post) =>
                <li key={post.id}>
                    {post.title}
                </li>
            )}
        </ul>
    );
    const content = props.posts.map((post) =>
        <div key={post.id}>
            <h3>{post.title}</h3>
            <p>{post.content}</p>
        </div>
    );
    return (
        <div>
            {sidebar}
            <hr />
            {content}
        </div>
    );
}

const posts = [
    { id: 1, title: 'Hello World', content: 'Welcome to learning React!' },
    { id: 2, title: 'Installation', content: 'You can install React from npm.' }
];
ReactDOM.render(
    <Blog posts={posts} />,
    document.getElementById('root')
);