React .map is not a function

Question

I'm trying to learn React and I'm a beginner when it comes to Javascript. Right now I'm working on an app that is fetching data from Flickr's API. The problem is that when I try to use the map method on the props in the Main.js component I get an error saying "Uncaught TypeError: this.props.photos.map is not a function". After searching here on Stackoverflow I think the problem is that this.props are javascript objects and not an array. The problem is that I can't figure out how to make it an array. Can anyone explain what I'm doing wrong?

My code:

class App extends Component {

  constructor() {
  super();
  this.state = {

  }
}

componentDidMount() {

let apiKey = 'xxxxxxxxxxxxxxxxxx';
let searchKeyword = 'nature';
let url = `https://api.flickr.com/services/ 
           rest/?api_key=${apiKey}&method=flickr.photos.
           search&format=json&nojsoncallback=1&&per_page=50
           &page=1&text=${searchKeyword}`;

fetch(url)
  .then(response => response.json())
  .then(data => data.photos.photo.map((x) => {

    this.setState({
      farm: x.farm,
      id: x.id,
      secret: x.secret,
      server: x.server})
   // console.log(this.state)
  }))
 }

    render() {
      return (
        <div className="App">
          <Header />
          <Main img={this.state.photos} />
          <Navigation />
        </div>
      );
    }
  }

  export default class Main extends Component {

  render() {

    return(
      <main className="main">
        {console.log(this.props.photos)}
      </main>
    )
  }
 }

Edit: Why is this.props.img undefined first?

Screen shot from console.log(this.props.img)

Answer 1

fetch(url)
  .then(response => response.json())
  .then(data => data.photos.photo.map((x) => {

    this.setState({
      farm: x.farm,
      id: x.id,
      secret: x.secret,
      server: x.server})
  }))

What is happening is that your map function in your promise is resetting the component's state for every photo that is returned. So your state will always be the last object in your list of returned photos.

Here is a more simplified example of what I am referring to

const testArray = [1,2,3,4];

let currentState;

testArray.map((value) => currentState = value)

console.log(currentState);

What you want to do is this

const testArray = [1,2,3,4];

let currentState;

//Notice we are using the return value of the map function itself.
currentState = testArray.map((value) => value)

console.log(currentState);

For what you are trying to accomplish, you want your state to be the result of the map function (since that returns an array of your results from the map). Something like this:

fetch(url)
  .then(response => response.json())
  .then(data => 
    this.setState({
      photos:
        data.photos.photo.map((x) => ({
          farm: x.farm,
          id: x.id,
          secret: x.secret,
          server: x.server
        }))
     })
   )

Answer 2

This error might also happen if you try to provide something else other than the array that .map() is expecting, even if you declare the variable type properly. A hook-based example:

const [myTwinkies, setMyTwinkies] = useState<Twinkies[]>([]);

useEffect(() => {
  // add a twinky if none are left in 7eleven
  // setMyTwinkies(twinkiesAt711 ?? {}); // ---> CAUSES ".map is not a function"
  setMyTwinkies(twinkiesAt711 ?? [{}]); 
}, [twinkiesAt711, setMyTwinkies]);

return (<ul>
  {myTwinkies.map((twinky, i)=> (
    <li key={i}>Twinky {i}: {twinky?.expiryDate}</li>
  ))}
</ul>)