How to have nested loops with map in JSX?

Answer 1

You need to wrap it inside an element.

Something like this (I've added an extra tr due to the rules of tables elements):

  render() {
    return (
      <table className="table">
        <tbody>
          {Object.keys(templates).map(function (template_name) {
            return (
              <tr key={template_name}>
                <tr>
                  <td>
                    <b>Template: {template_name}</b>
                  </td>
                </tr>
                {templates[template_name].items.map(function (item) {
                  return (
                    <tr key={item.id}>
                      <td>{item}</td>
                    </tr>
                  );
                })}
              </tr>
            );
          })}
        </tbody>
      </table>
    );
  }
}

Running Example (without a table):

const templates = {
  template1: {
    items: [1, 2]
  },
  template2: {
    items: [2, 3, 4]
  },
};

const App = () => (
  <div>
    {
      Object.keys(templates).map(template_name => {
        return (
          <div>
            <div>{template_name}</div>
            {
              templates[template_name].items.map(item => {
                return(<div>{item}</div>)
              })
            }
          </div>
        )
      })
    }
  </div>
);
ReactDOM.render(<App />, document.getElementById('root'));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="root"></div>

Answer 2

I struggled for a while to get my nested map function to work only to discover that what you return is critical. Make sure you are returning the second map itself and not just the final expected output:

let { categories } = data;

categories = categories.map(category =>
    category.subcategories.map((subcategory, i) => <h2 key={i}>{subcategory.name}</h2>)
);

return (
    <div className="category-container">
        <div>{categories}</div>
    </div>
);

Answer 3

I think the problem is that the return type should be an array but not an object in React16. You could try like this below:

class App extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
      templates: {
        foo: {
          items: [
            {id: 0},{id:1}
          ]
        },
        bar: {
          items: [
            {id: 2},{id:3}
          ]
        }
      }
    }
  }
  
  renderTemplate = (template, name) => {
    let data = []
    data = template.items
    data.unshift({ name: name })
    return data.map((item, index) => <tr key={index}><td>{item.name ? item.name : item.id}</td></tr>)
  }
  
  render() {
    return (
      <table>
        <tbody>
          {Object.keys(this.state.templates).map(name => {
            return this.renderTemplate(this.state.templates[name], name)
          })}
        </tbody>
      </table>
    )
  }
}



ReactDOM.render(<App />, document.getElementById('root'))

td {
  color: white;
  padding: 0 20px;
  background: grey;
}

<div id="root"></div>

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.1.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.1.1/umd/react-dom.production.min.js"></script>