each child in a list should have a unique 22key 22 prop

1<ul>
2  {["Item1", "Item2", "Item3"].map(item =>
3  <li key="{item}">{item}</li>
4  )}
5</ul>
6

1const numbers = [1, 2, 3, 4, 5];
2const listItems = numbers.map((number) =>
3  <li key={number.toString()}>    {number}
4  </li>
5);

1<ul>
2  {["Item1", "Item2", "Item3"].map(item =>
3  <li>{item}</li>
4  )}
5</ul>
6

1When creating a list in the UI from an array with JSX, you should add a key prop to each child and to any of its’ children.
2
3Ex: <li key="uniqueId1" >Item1</li>
4
5React uses the key prop create a relationship between the component and the DOM element. The library uses this relationship to determine whether or not the component should be re-rendered.
6
7It is not recommended to use the index of the array as the key prop if you know the array will not be static. If the key is an index, reordering an item in the array changes it. Then React will get confused and re-render the incorrect element.
8
9<--Important-->
10Keys do not have to be unique globally. They just need to be unique across sibling elements.
11
12<ul>
13  {["Item1", "Item2", "Item3"].map(item =>
14  <li key="{item}">{item}</li>
15  )}
16</ul>

1<ul>
2  {["Item1", "Item2", "Item3"].map(item =>
3  <li key="{item}">{item}</li>
4  )}
5</ul>

1Warning: Each child in a list should have a unique "key" prop
2