A valid React element (or null) must be returned. You may have returned undefined, an array or some other invalid object

Question

I'm just new in ReactJS and I have a problem. I can't solve it. It seems everything is all right, but still console puts me:

A valid React element (or null) must be returned. You may have returned undefined, an array or some other invalid object.

Here's my code:

import React from 'react';
import ReactDOM from 'react-dom';
import fetch from 'isomorphic-fetch';
import Pokemon from './Pokemon';

class PokemonList extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      species: [],
      fetched: false,
      loading: false,
    };
  }
  componentWillMount(){
    this.setState({
      loading : true
    });
    fetch('http://pokeapi.co/api/v2/pokemon?limit=151').then(res => res.json())
    .then(res =>{
      this.setState({
        species : res.results,
        loading : true,
        fetched : true
      });
    });
  }
  render() {
    const {fetched, loading, species} = this.state;
    let content;
    //This if seems to be the problem
    if(fetched){
      content =
      <div className="pokemon--species--list">
        {species.map((pokemon,index) => <Pokemon key={pokemon.name} id={index+1} pokemon={pokemon}/>)}
      </div>;
    }
    else if(loading && !fetched){
        content = <p> Loading ...</p>;
    }
    else{
      content = <div/>;
    }
    return (
      <div>
        {content}
      </div>
    );
  }
}

export default PokemonList;

Pokemon.js

import React from 'react';
import ReactDOM from 'react-dom';


class Pokemon extends React.Component {
  render() {
    const {pokemon, id} = this.props;
    return
      <div className="pokemon--spacies">
        <div className="pokemon--spacies--container">
          <div className="pokemon--spacies--sprite">
            <img src={`/public/sprites/${id}.png`} />
          </div>
          <div className="pokemon--spacies--name"> {pokemon.name }</div>
        </div>
      </div>;
  }
}

export default Pokemon;

Thanks for help!

Answer 1

The problem is that you have multiple statements in render and hence you need to surround that within () See the below snippet. Also the ( must be on the same line as return otherwise it will be treated a return only which is basically returning nothing.

class Pokemon extends React.Component {
  render() {
    var content = <div></div>
    return (
      <div className="pokemon--spacies">
        <div className="pokemon--spacies--container">
          <div className="pokemon--spacies--sprite">
            {content}
          </div>
          <div className="pokemon--spacies--name"> Hello</div>
        </div>
      </div>
    )
  }
}
ReactDOM.render(<Pokemon/>, document.getElementById('app'));

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

Answer 2

Problem is in the return statement in Pokemon component, Because when you use:

return
   <div>
     ...
   </div>

It will be treated as:

return ;    //Automatic semicolon insertion

<div>
   ...
</div>

And that means you are not returning a valid element.

Check this answer for more details about Automatic Semicolon Insertion.

Solutions:

Wither wrap all the elements by () like this:

return (
 ....
)

or put the div in the same line of return, like this:

return <div>
   ...
       </div>

Use this part it will work:

class Pokemon extends React.Component {
  render() {
    const {pokemon, id} = this.props;
    return(
      <div className="pokemon--spacies">
        <div className="pokemon--spacies--container">
          <div className="pokemon--spacies--sprite">
            <img src={`/public/sprites/${id}.png`} />
          </div>
          <div className="pokemon--spacies--name"> {pokemon.name }</div>
        </div>
      </div>);
  }
}