TypeScript Readonly and Immutability

Immutability prevents accidental mutations. Here's how TypeScript helps enforce it.

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

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.

TypeScript Readonly and Immutability

Readonly Properties#

Readonly Type#

ReadonlyArray#

Const Assertions#

Deep Readonly#

Immutable Patterns#

Readonly vs Immutable#

Readonly in Functions#

Mutable vs Readonly Types#

Immutable Libraries#

Best Practices#

Conclusion#

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