Warning: Each Child in a List Should Have a Unique 'key' Prop

Nisanthan Nanthakumar

The Problem

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

<ul>
  {["Item1", "Item2", "Item3"].map(item =>
  <li>{item}</li>
  )}
</ul>

The Solution

When 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.

Ex: <li key="uniqueId1" >Item1</li>

React 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.

It 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.

Keys do not have to be unique globally. They just need to be unique across sibling elements.

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

Further Reading

If you’re looking to get a deeper understanding of how JavaScript application monitoring works, take a look at the following articles:

• Capture and report JavaScript errors with window.onerror
• 4 reasons why your source maps are broken
• Getting started with a JavaScript SDK (docs)