# Reflect.2. Deriving props — Mercury LMS

> Contract: `@fiberfx/reflect` 4.1.1; Hypergraph and Hyperscript 4.1.1.

## Contract

mapProps is a config field, not an export. Sources are Store, flat object/tuple of stores, or empty object. fn(values, baseProps) is pure and synchronous. All projections receive bound/incoming props, never sibling outputs. Mapped keys are optional in that input type.

## Overrides and refusals

An own prop, including explicit undefined, skips the matching fn. Omission retains derivation. bind/mapProps overlap throws; non-store and nested sources refuse. Do not infer subscription elimination from projection suppression. Returned callables are unchanged and unscoped.

## Exercise

[Projection lab](/examples/reflect-lab/index.html?part=projections&lang=en): name and prefix, fixed override, undefined override, remove override, reset. Transfer to a count-based status line. Optional binding keys do not guarantee required props; prefer satisfies when preserving literal keys.

## Source

`fiberfx/reflect/src/reflect.ts:32-62`, `src/types.ts:47-63`, `test/collections.test.ts:24-88`; npm 4.1.1.

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