flow-not-serializable

[wizzard] steps.<id>.when is a function. JSON.stringify drops a function, so the flow would not
survive being stored or sent. Move the function into the registry and name it with a $ref.
…/errors/flow-not-serializable

Returned by validateFlow as a problem whose path is where the function is, such as steps.<id>.when, and listed in the message of flow-invalid by assertFlow.

A flow is JSON, so it can be stored, diffed, sent by a server and drawn by the inspector. JSON.stringify drops a function without a word, so a flow with one in it stops round-tripping: what arrives on the other side is a different flow, missing a predicate or a validator.

Everything a flow cannot carry is named instead. Move the function into the registry and refer to it with a $ref:

const flow = defineFlow({
  id: 'signup',
  steps: { company: { when: { $ref: 'isBusiness' } } },
});

const wizard = createWizard({
  flow,
  registry: { isBusiness: (_args, { data }) => data.account?.type === 'business' },
});

The same code is returned for an object that contains itself, with the message <path> contains itself. JSON.stringify throws on a cycle rather than dropping it, so such a flow cannot be stored or sent at all; replace the reference with a copy of the value. An object used in two places is not a cycle and is not reported.

Both are found at any depth anywhere in the flow - in ui and in a $ref’s args as much as in an expression, and in validate, policy or any other field beside steps - because this check, unlike the expr-too-deep one, has no nesting limit.

A condition that only compares values needs no resolver; write it with the operators in the expressions guide.