Error Handling

Type Safety

TESM provides strict type checking at compile time. When creating a state machine using defineFlow(), TypeScript verifies that:

1. All states from Model have corresponding handlers in flow
2. All handlers in flow correspond to existing states
3. All messages in handlers correspond to defined Msg

Runtime Errors

When a message cannot be handled in the current state, TESM will throw an exception. This behavior can be modified using onInvalidState.

import { invalidStateMsg } from "tesm"

const machine = defineFlow(m, "MachineName", initial, flow, extras, {
    onInvalidState: (machine, msg, model) => {
        console.error(`Invalid state transition in ${machine}: ${model.state}.${msg.type}`);
    },
    // or use built-in error message generator
    onInvalidState: (machine, msg, model) => {
        console.error(invalidStateMsg(machine, msg, model));
    }
});

The onInvalidState handler receives:

• machine: State machine name (second argument of defineFlow)
• msg: Message that cannot be handled
• model: Current state model

Universal Message Handlers

The extras parameter of the defineFlow() function allows you to define message handlers that will be called if the current state doesn't have its own handler for that message.

const machine = defineFlow(m, "MachineName", initial, flow, {
    some_message: (msg, model) => [
        m.states.some_state({}),
        m.cmds.some_cmd()
    ],
});

Ignoring Transition

In machine there is a helper ignore that returns current model, so transition is simply ignored

const machine = defineFlow(m, "MachineName", initial, flow, {
    other_message: m.ignore,
});