Javascript

//index.html
<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
	<div id = "root"></div>
	<script src="https://fb.me/react-0.14.3.js"></script>
	<script src="https://fb.me/react-dom-0.14.3.js"></script>
	<script src="https://unpkg.com/babel-standalone@6.26.0/babel.min.js"></script>
	<script src="script.js" type = "text/babel"></script>
</body>
</html>

//script.js
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')
);

Keys help React identify which items have changed, are added, or are removed

Keys should be given to the elements inside the array to give the elements a stable identity

keep the key on the <ListItem /> elements in the array rather than on the <li> element in the ListItem itself

Keys used within arrays should be unique among their siblings, however they don’t need to be globally unique

Keys serve as a hint to React but they don’t get passed to your components

function ListItem(props) {
  // Correct! There is no need to specify the key here:
  return <li>{props.value}</li>;
}

function NumberList(props) {
  const numbers = props.numbers;
  const listItems = numbers.map((number) =>
    // Correct! Key should be specified inside the array.
    <ListItem key={number.toString()}
              value={number} />

  );
  return (
    <ul>
      {listItems}
    </ul>
  );
}

const numbers = [1, 2, 3, 4, 5];
ReactDOM.render(
  <NumberList numbers={numbers} />,
  document.getElementById('root')
);