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A error message in React console

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

Friday, November 27, 2020

At 235 words, this article should take about 1 minute to read.

To prevent ugly errors in your console when you loop through an array, React likes you to use a unique key for each child element.

We usually use the loop index. This is not advised for several reasons1, 2.

Instead try this…

Math.random().toString(36).substr(2, 9)

This will give you a (fairly) random 9-character alphanumerical string.

Example code
<ul>
    { items.map(x => <li key={ Math.random().toString(36).substr(2, 9) }>{x}</li> }
</ul>

This is useful for "throwaway" keys. If you're going to be referencing the keys in any way, you need to use a unique property (like an ID or slug).


1 React Docs say so

2 Stack Overflow Bros say so

Comments

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Thomas Banks

Thomas Banks

When I'm not building things for the internet, I take photos of stuff.
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