Models

In XState, you can model a machine's context and events externally by using createModel(...). This provides a convenient way to strongly type context and events, as well as helpers for event creation, assignment and other implementation details in the future.

Using createModel(...) is completely optional, and is meant to improve the developer experience. The main reasons for using it are:

• Separating and organizing context and events in a strongly-typed way
• Preventing typing issues with assign(...)
• Specifying event creators for easier and safer event creation
• Potentially sharing the model with other machines
• Future developer experience improvements, such as specifying actions, guards, etc.

createModel(...)

The createModel(...) function takes

Argument	Type	Description
initialContext	object	The initial context value
creators (optional)	object	An object containing various event creators

The creators object includes the following properties:

Argument	Type	Description
events	object	An object containing event creators

The keys of the creators.events object are event types, and the values are functions that accept any number of arguments and return the event payload.

Modeling context

Since the model defines the machine's context, the model can be used within the machine definition to set its initial context with model.initialContext and to update the machine's context with model.assign.

The model.assign function is typed to the shape of the model's context, making it a convenient and type-safe replacement for the assign action.

import { createModel } from 'xstate/lib/model';

const userModel = createModel({
  name: 'Someone',
  age: 0
});

// ...

const machine = userModel.createMachine({
  context: userModel.initialContext,
  // ...
  entry: userModel.assign({ name: '' })
});

Modeling events

Modeling machine events in a model gives two benefits:

• Events can be created by calling model.events.eventName(...)
• Provides type information to the machine definition, providing event-specific type safety for action definitions

import { createModel } from 'xstate/lib/model';

const userModel = createModel(
  // Initial context
  {
    name: 'David',
    age: 30
  },
  {
    // Event creators
    events: {
      updateName: (value) => ({ value }),
      updateAge: (value) => ({ value }),
      anotherEvent: () => ({}) // no payload
    }
  }
);

const machine = userModel.createMachine(
  {
    context: userModel.initialContext,
    initial: 'active',
    states: {
      active: {
        on: {
          updateName: {
            actions: userModel.assign({
              name: (_, event) => event.value
            })
          },
          updateAge: {
            actions: 'assignAge'
          }
        }
      }
    }
  },
  {
    actions: {
      assignAge: userModel.assign({
        age: (_, event) => event.value // inferred
      })
    }
  }
);

// This sends the following event:
// {
//   type: 'updateName',
//   value: 'David'
// }
const nextState = machine.transition(
  undefined,
  userModel.events.updateName('David')
);

TypeScript

The createModel(...) function infers the following types:

• context is inferred from the first argument in createModel(initialContext, creators)
• events is inferred from creators.events in createModel(initialContext, creators)

import { createModel } from 'xstate/lib/model';

const userModel = createModel(
  {
    name: 'David', // inferred as `string`
    age: 30, // inferred as `number`
    friends: [] as string[] // explicit type
  },
  {
    events: {
      updateName: (value: string) => ({ value }),
      updateAge: (value: number) => ({ value }),
      anotherEvent: () => ({}) // no payload
    }
  }
);

// Context inferred as:
// {
//   name: string;
//   age: number;
//   friends: string[];
// }

// Events inferred as:
// | { type: 'updateName'; value: string; }
// | { type: 'updateAge'; value: number; }
// | { type: 'anotherEvent'; }

Creating a machine from a model

Instead of specifying the type of context and event explicitly as type parameters in createMachine<TContext, TEvent>(...), the model.createMachine(...) method should be used:

const machine = userModel.createMachine({
  context: userModel.initialContext,
  initial: 'active',
  states: {
    active: {
      on: {
        updateName: {
          actions: userModel.assign({
            name: (_, event) => event.value // inferred
          })
        }
      }
    }
  }
});

Narrowing assign event types

When an assign() action is referenced in options.actions, you can narrow the event type that the action accepts in the 2nd argument of model.assign(assignments, eventType):

const assignAge = userModel.assign(
  {
    // The `event.type` here is restricted to "updateAge"
    age: (_, event) => event.value // inferred as `number`
  },
  'updateAge' // Restricts the `event` allowed by the "assignAge" action
);

const machine = userModel.createMachine({
  context: userModel.initialContext,
  initial: 'active',
  states: {
    active: {
      on: {
        updateAge: {
          actions: assignAge
        }
      }
    }
  }
});

WARNING

Assign actions with narrowed event types cannot be placed inside the actions: {...} property of machine options in createMachine(configuration, options). This is because actions in options.actions should be assumed to potentially receive any event, even if the machine configuration suggests otherwise.

Extracting types from model

Since 4.22.1

You can extract context and event types from a model using the ContextFrom<T> and EventFrom<T> types:

import { ContextFrom, EventFrom } from 'xstate';
import { createModel } from 'xstate/lib/model';

const someModel = createModel(
  {
    /* ... */
  },
  {
    events: {
      /* ... */
    }
  }
);

type SomeContext = ContextFrom<typeof someModel>;
type SomeEvent = EventFrom<typeof someModel>;