function NumberList(props) {
    const numbers = props.numbers;
    const listItems = numbers.map((number) =>
      <li key={number.toString()}>
        {number}
      </li>
    );
    return (
      <ul>{listItems}</ul>
    );
  }
  
  const numbers = [1, 2, 3, 4, 5];
  ReactDOM.render(
    
 
<NumberList numbers={numbers} />,
    document.getElementById(‘root‘)
  );

//key值最好選用id，沒有穩定的id的時候選擇index
 const todoItems = todos.map((todo) =>
    <li key={todo.id/index}>
        {todo.text}
    </li>
);

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‘)
  );

react 中文文檔案例五 （循環列表）