TypeScript provides tools for enforcing immutability at the type level. Here's how to use them effectively.
Readonly Properties
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Readonly<T> Utility Type
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ReadonlyArray<T>
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Const Assertions
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Object Literals with as const
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Enum-like Constants
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Tuple Types
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Readonly in Functions
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Satisfies with as const
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Readonly Maps and Sets
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Mutable from Readonly
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Patterns and Use Cases
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Best Practices
When to Use Readonly:
✓ Configuration objects
✓ Function parameters you won't modify
✓ Redux/state management
✓ Public API return types
When to Use as const:
✓ Object literal constants
✓ Array constants (enum-like)
✓ Preserving literal types
✓ Tuple creation
Patterns:
✓ Readonly for interfaces
✓ as const for values
✓ DeepReadonly for nested
✓ Mutable when needed
Avoid:
✗ Readonly on primitives (unnecessary)
✗ Over-using DeepReadonly
✗ Forgetting shallow nature
✗ Mixing mutable and readonly
Conclusion
TypeScript's readonly modifiers and const assertions provide compile-time immutability guarantees. Use readonly for interface properties, Readonly<T> for entire types, and as const for literal value preservation. Remember that readonly is shallow by default - use DeepReadonly for nested immutability. These tools help prevent accidental mutations and make code intent clearer.