In short

• Write plain constants in components; they are live when they read signals or props and are used in JSX or a closure.
• Use useMemo outside components (hooks, modules), for values you want to name, and whenever the constant is not at the top level of the component body.
• Use <Show>, <Switch> or one returned conditional for branching; never an early return that depends on a signal.
• Do not reuse a derived constant's name in an inner scope.

See also the mental model for what a component body does once, and Configuration for autoReactive.