TypeScript Readonly and Const Assertions

TypeScript provides tools for enforcing immutability at the type level. Here's how to use them effectively.

Object Literals with as const#

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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

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.

TypeScript Readonly and Const Assertions

Readonly Properties#

Readonly<T> Utility Type#

ReadonlyArray<T>#

Const Assertions#

Object Literals with as const#

Enum-like Constants#

Tuple Types#

Readonly in Functions#

Satisfies with as const#

Readonly Maps and Sets#

Mutable from Readonly#

Patterns and Use Cases#

Best Practices#

Conclusion#

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