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My checkout form can reach a screen where the payment step shows before a plan was ever picked, and I don't know how that happened.
What you'll have at the end
A state chart that can't reach the payment step without a selected plan, tested against the old bug
You need
A checkout flow already split into separate screens for a plan, billing details, and payment, currently tracking which screen shows next with a raw step number or a handful of independent boolean flags instead of one value that describes the whole wizard.
Not covered
Validating what's actually typed into each screen, such as a malformed address or a bad card number; this only decides which screen the wizard can reach next, not whether what's already in it is valid enough to submit.
Leans on
Force an exhaustive switch that breaks the build on a missed case
for the fuller walkthrough of the never-typed check this recipe's own render step only applies
Make a save action optimistic with TanStack Query
when submitting payment is itself a server call that should show progress before it resolves, rather than only guarding which screen can appear next
Checked 22 Aug 2026
Part of the Frontend State Management Beyond Redux cookbook