Disallow missing `key` prop in JSX elements that likely require a `key` prop (react/jsx-key)

[feross]

https://github.com/yannickcr/eslint-plugin-react/blob/master/docs/rules/jsx-key.md

This rule caught a ton of instances where I forgot key in bitmidi.com, where I use Preact and the development tools don't seem to warn about this.

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Warn if an element that likely requires a key prop--namely, one present in an
array literal or an arrow function expression.

Rule Details

The following patterns are considered warnings:

[<Hello />, <Hello />, <Hello />];

data.map(x => <Hello>{x}</Hello>);

<Hello {...{ key: id, id, caption }} />

In the last example the key is being spread, which is currently possible, but discouraged in favor of the statically provided key.

The following patterns are not considered warnings:

[<Hello key="first" />, <Hello key="second" />, <Hello key="third" />];

data.map((x, i) => <Hello key={i}>{x}</Hello>);

<Hello key={id} {...{ id, caption }} />

[feross]

Shipping in standard 14.