Add changeset

.changeset/rich-terms-lick.md

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+---
+'xstate': minor
+---
+
+An opt-in `createModel()` helper has been introduced to make it easier to work with typed `context` and events.
+
+- `createModel(initialContext)` creates a `model` object
+- `model.initialContext` returns the `initialContext`
+- `model.assign(assigner, event?)` creates an `assign` action that is properly scoped to the `event` in TypeScript
+
+See https://github.com/davidkpiano/xstate/pull/1439 for more details.
+
+```js
+import { createMachine } from 'xstate';
+import { createModel } from 'xstate/lib/model'; // opt-in, not part of main build
+
+interface UserContext {
+  name: string;
+  age: number;
+}
+
+type UserEvents =
+  | { type: 'updateName'; value: string }
+  | { type: 'updateAge'; value: number }
+
+const userModel = createModel<UserContext, UserEvents>({
+  name: 'David',
+  age: 30
+});
+
+const assignName = userModel.assign({
+  name: (_, e) => e.value // correctly typed to `string`
+}, 'updateName'); // restrict to 'updateName' event
+
+const machine = createMachine<UserContext, UserEvents>({
+  context: userModel.context,
+  initial: 'active',
+  states: {
+    active: {
+      on: {
+        updateName: {
+          actions: assignName
+        }
+      }
+    }
+  }
+});
+```