Welcome to the Treehouse Community

Want to collaborate on code errors? Have bugs you need feedback on? Looking for an extra set of eyes on your latest project? Get support with fellow developers, designers, and programmers of all backgrounds and skill levels here with the Treehouse Community! While you're at it, check out some resources Treehouse students have shared here.

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and join thousands of Treehouse students and alumni in the community today.

Start your free trial

JavaScript React Basics (2018) Introducing Props Iterating and Rendering with map()

Posted by hun ee4
hun ee4
161 Points

During Iterating and Rendering with map() ,why and where it comes from ? why typing props.initialPlayers.map

why typing props.initialPlayers.map , not props.Players.map

<Player name="Guil" score={50} />
<Player name="Treasure" score={90} />
<Player name="Ashley" score={85} />
<Player name="James" score={80} />

 {props.initialPlayers.map( player =>
<Player 
name={player.name}
score={player.score}  
 />
)}

2 Answers

Ezra Siton
Ezra Siton
12,644 Points
Ezra Siton
Ezra Siton
12,644 Points

initialPlayers is the name of the prop VS Players - is the value of initialPlayers (Could be "hello", "55" or "bla bla" or const with an array of objects).

Take a look at this example (I put list-of-objects directly inside initialPlayers instead of const):

ReactDOM.render(
  <App initialPlayers={[{name: "john"}, {name: "ben"}]} />,
  document.getElementById('root')
);

The map() method creates a new array with the results of calling a provided function on every element in the calling array. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map

You want to loop throw the array (Otherwise you get this error):

Uncaught TypeError: Cannot read property 'map' of undefined

One more Very simple example:

const soccerPlayers = [
  {
    name: "Ronaldo",
    score: 100
  },
  {
    name: "Messi",
    score: 100
  },
  {
    name: "Bale",
    score: 90
  }
];


const App = (props) => {
  return (
    <ul>
      {/* Players list */}
    {props.initialPlayers.map( player =>
        <li>{player.name}: {player.score} </li>    
  )}
    </ul>
  );
}

ReactDOM.render(
  <App initialPlayers={soccerPlayers}  />,
  document.getElementById('root')
);
marcell gibbs
marcell gibbs
3,744 Points
marcell gibbs
marcell gibbs
3,744 Points

i am getting that error even while using map

Everton Carneiro
Everton Carneiro
15,994 Points
Everton Carneiro
Everton Carneiro
15,994 Points

Because initialPlayers is the props of the App component.