Immutability prevents accidental mutations. Here's how TypeScript helps enforce it.
Readonly Properties
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Readonly Type
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ReadonlyArray
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Const Assertions
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Deep Readonly
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Immutable Patterns
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Readonly vs Immutable
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Readonly in Functions
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Mutable vs Readonly Types
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Immutable Libraries
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Best Practices
Design:
✓ Default to readonly for data
✓ Use const assertions for constants
✓ Make function params readonly
✓ Return readonly from getters
Patterns:
✓ Spread for immutable updates
✓ map/filter/reduce for arrays
✓ Use Immer for complex updates
✓ Deep readonly for nested data
Performance:
✓ Readonly has no runtime cost
✓ Object.freeze has minimal cost
✓ Structural sharing in libraries
✓ Avoid deep cloning when possible
Avoid:
✗ Type assertions to bypass readonly
✗ Shallow readonly for nested data
✗ Mutating "readonly" at runtime
✗ Over-freezing in hot paths
Conclusion
TypeScript's readonly features catch mutation errors at compile time. Use Readonly<T> for shallow protection, deep readonly types for nested data, and const assertions for literal types. Combine with runtime immutability (Object.freeze or libraries) when true immutability is required.