Derived Values

Some values aren’t state of their own — they’re computed from state. A doubled count. A filtered list. A total. For those, Marko has <const>:

<let/count=1>
<const/doubleCount=count * 2>

Whenever count changes, doubleCount recomputes. You never assign to it — it’s derived, always correct, impossible to forget to update. That’s the division of labor: <let> for values you change, <const> for values that follow.

(Don’t confuse it with static const from part one: static runs once when the template loads and can’t react to anything. <const> is per-instance and lives inside the reactive graph.)

The page on the right sells tickets and does its math inline — the price appears in two places, written out twice, one of them already wrong. Fix the duplication:

1. Under the <let> declarations, derive once: <const/total=tickets * PRICE>.
2. Replace both inline calculations with ${total} — including the broken one.

Now bump the ticket count: one derivation, both places correct, forever.