RMD019 — State set to a value that cannot be serialized

@state travels to the client in the hydration blob as JSON, so it can only hold JSON-serializable data. Assigning a function, a symbol, or a bigint to a @state field is flagged the moment it happens — at the field initializer (@state x = …) or a later write — because JSON has no way to carry it: a function and a symbol are dropped silently, a bigint makes serialization throw. Either way the client would come alive with that field missing.

@state onPick = () => {};   // ✗ a function — not serializable
@state total = 42n;         // ✗ a bigint

Keep behaviour off state: a function is a method on the class, or a prop passed in (props are not serialized — the parent re-supplies them on the client). A symbol or bigint should be a string or number in state. And if the field is genuinely client-only and never meant to travel, it should be a plain field, not @state. Deeper cases — a Map, a Date, a circular object — are caught by the server renderer when it serializes, rather than at the write.

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RMD019 — State set to a value that cannot be serialized

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