jsx-key

Keywords: React, JSX, key prop, reconciliation, lists, map, iteration, ESLint rule, performance, LLM-optimized

Detect missing or problematic React keys that could break reconciliation. This rule is part of eslint-plugin-react-features and provides LLM-optimized error messages with suggestions.

Quick Summary

Aspect	Details
Severity	Error (correctness)
Auto-Fix	💡 Suggests fixes
Category	React
ESLint MCP	✅ Optimized for ESLint MCP integration
Best For	All React/JSX projects

Rule Details

Why This Matters

Issue	Impact	Solution
🔄 Reconciliation	React can't track elements	Add unique keys
⚡ Performance	Unnecessary re-renders	Stable keys
🐛 State Bugs	Wrong component gets state	Use item IDs, not indexes
🎨 Animation Issues	Elements animate incorrectly	Consistent key identity

Configuration

Option	Type	Default	Description
`warnUnstableKeys`	`boolean`	`true`	Warn about potentially unstable keys

Examples

❌ Incorrect

// Missing key
function UserList({ users }) {
  return (
    <ul>
      {users.map(user => (
        <li>{user.name}</li>  // ❌ Missing key
      ))}
    </ul>
  );
}

// Using index as key (unstable)
function ItemList({ items }) {
  return (
    <ul>
      {items.map((item, index) => (
        <li key={index}>{item.name}</li>  // ⚠️ Unstable key
      ))}
    </ul>
  );
}

✅ Correct

// Using unique ID as key
function UserList({ users }) {
  return (
    <ul>
      {users.map(user => (
        <li key={user.id}>{user.name}</li>  // ✅ Stable unique key
      ))}
    </ul>
  );
}

// Using compound key when no ID available
function ItemList({ items }) {
  return (
    <ul>
      {items.map(item => (
        <li key={`${item.category}-${item.name}`}>
          {item.name}
        </li>  // ✅ Unique compound key
      ))}
    </ul>
  );
}

Configuration Examples

Basic Usage

{
  rules: {
    'react-features/jsx-key': 'error'
  }
}

Disable Unstable Key Warnings

{
  rules: {
    'react-features/jsx-key': ['error', {
      warnUnstableKeys: false
    }]
  }
}

Key Best Practices

Choosing Keys

Source	Quality	Example
Database ID	✅ Best	`key={user.id}`
Unique field	✅ Good	`key={item.slug}`
Compound unique	✅ Good	`key={cat + '-' + name}`
UUID/nanoid	⚠️ OK	`key={generateId()}`
Array index	❌ Avoid	`key={index}`
Random number	❌ Never	`key={Math.random()}`

When Index Keys Are Acceptable

// ✅ Static list that never reorders/filters
const WEEKDAYS = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri'];
<ul>
  {WEEKDAYS.map((day, i) => <li key={i}>{day}</li>)}
</ul>

// ❌ Dynamic list - DON'T use index
{users.map((user, i) => <UserCard key={i} user={user} />)}  // Bad!
{users.map(user => <UserCard key={user.id} user={user} />)} // Good!

Common Patterns

Fragment Keys

// When using fragments in lists, key goes on Fragment
function DataList({ data }) {
  return (
    <>
      {data.map(item => (
        <React.Fragment key={item.id}>
          <dt>{item.term}</dt>
          <dd>{item.definition}</dd>
        </React.Fragment>
      ))}
    </>
  );
}

Nested Lists

function NestedList({ categories }) {
  return categories.map(category => (
    <div key={category.id}>
      <h2>{category.name}</h2>
      <ul>
        {category.items.map(item => (
          <li key={item.id}>{item.name}</li>  // Separate key namespace
        ))}
      </ul>
    </div>
  ));
}

When Not To Use

Scenario	Recommendation
🧪 Testing/Prototypes	Consider allowing index keys temporarily
📊 Static content	Index keys may be acceptable
🔄 No reordering	Index keys work for append-only lists

Comparison with Alternatives

Feature	jsx-key	eslint-plugin-react	jsx-ally
Missing key	✅ Yes	✅ Yes	❌ No
Unstable keys	✅ Configurable	⚠️ Limited	❌ No
LLM-Optimized	✅ Yes	❌ No	❌ No
ESLint MCP	✅ Optimized	❌ No	❌ No
Suggestions	✅ Yes	⚠️ Limited	❌ No

react-no-inline-functions - Performance optimization
react-render-optimization - Render performance

Known False Negatives

The following patterns are not detected due to static analysis limitations:

Dynamic Variable References

Why: Static analysis cannot trace values stored in variables or passed through function parameters.

// ❌ NOT DETECTED - Prop from variable
const propValue = computedValue;
<Component prop={propValue} /> // Computation not analyzed

Mitigation: Implement runtime validation and review code manually. Consider using TypeScript branded types for validated inputs.

Imported Values

Why: When values come from imports, the rule cannot analyze their origin or construction.

// ❌ NOT DETECTED - Value from import
import { getValue } from './helpers';
processValue(getValue()); // Cross-file not tracked

Mitigation: Ensure imported values follow the same constraints. Use TypeScript for type safety.

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