Index signatures allow typing objects with dynamic keys. Here's how to use them effectively.
Basic Syntax
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With Known Properties
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Record Type
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Mapped Types vs Index Signatures
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Generic Dictionaries
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Safe Index Access
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Template Literal Keys
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Readonly Index Signatures
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Discriminated Index Types
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Object.keys and Index Signatures
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Symbol Index Signatures
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Class with Index Signature
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Validation with Index Signatures
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Best Practices
When to Use:
✓ Dynamic object keys
✓ Dictionary/map patterns
✓ Configuration objects
✓ Cache implementations
Safety:
✓ Enable noUncheckedIndexedAccess
✓ Check for undefined
✓ Use Map for runtime safety
✓ Prefer Record<K, V> syntax
Patterns:
✓ Combine with known properties
✓ Use template literal keys
✓ Consider readonly
✓ Use generics for flexibility
Avoid:
✗ Overly permissive signatures
✗ Ignoring undefined returns
✗ Complex nested signatures
✗ When specific keys are known
Conclusion
Index signatures type objects with dynamic keys, enabling dictionary patterns and flexible configurations. Use [key: string]: Type for string keys, combine with known properties when needed, and prefer Record<K, V> for cleaner syntax. Enable noUncheckedIndexedAccess for safety, as index access can return undefined. For runtime type safety with dynamic keys, consider using Map instead of plain objects.