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immutable-patterns

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skills/immutable-patterns/SKILL.md

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# Prefer immutable data patterns

Mutating shared objects makes it impossible to track where a value changed. When you update state immutably, each version of the data is a separate object — you can compare references to detect changes, time-travel debug, and know exactly which part of your code changed the data.

## Quick Reference

- Use spread (...) to create modified copies of objects and arrays
- Prefer map(), filter(), and reduce() over push(), splice(), and direct assignment
- Use Object.freeze() for configuration objects that should never change
- Immutability is especially important in state management (Redux, Zustand, Signals)

## Check

Find all places in this code where objects or arrays are mutated in place. Flag direct property assignments on function parameters and push/splice on arrays.