expr-unknown-operator

[wizzard] "<key>" is not an operator. An expression object names its operation with a key, and
none of its keys is an operator. Replace <key> with an operator from the expressions guide, or
check it for a typo. …/errors/expr-unknown-operator

Thrown as a WizardError by evaluate or evaluateAsync, and op says which. <key> is the object’s first key, or {} for an empty object.

Not every expression lets it out. In a when or a transition’s when the engine catches it: the step is read as not reachable and the line is logged as when-threw. In a guard it stops the navigation, since a guard decides one move and its caller is waiting. In a repeat group’s repeat.over and in input it is caught too: the group is read as having no items, and the input as undefined. Only a guard throws to a caller; elsewhere the typo shows as a console line, and validateFlow finds it before anything runs at all.

validateFlow finds the same objects before anything is evaluated, in every place the engine reads an expression - when, guards, a transition’s when, repeat.over and input - and returns each as a problem with code: 'expr-unknown-operator', the path of the object, and the message above. The ui of a step and the args of a $ref are data rather than expressions, and are not read.

An expression is JSON: a literal, a list, or an object whose key names the operation - $get, $eq, $and, $ref and the rest in the expressions guide. The evaluator looks for one of those keys, and when the object has none it throws rather than guessing, because an expression that evaluates to undefined would quietly read as false in a when.

The usual causes are a typo ($equals for $eq), a key without its $, an operator from another expression language, or a flow written for a newer version of the library than the one installed. An object meant as a literal value cannot be written inline, since its keys would be read as an operator; put it in data or ctx and read it with $get.

defineFlow and the expr builder from @wizzard-packages/core/expr produce only operators that exist, which is the easiest way never to see this for a flow written in code. A flow that arrives from a server is the case this error is for.