Add type declaration tests

package.json

@@ -10,7 +10,9 @@
   },
   "types": "index.d.ts",
   "scripts": {
-    "test": "mocha"
+    "test": "npm run test:units && npm run test:types",
+    "test:units": "mocha",
+    "test:types": "tsc --noEmit test/*.ts"
   },
   "engines": {
     "node": ">=14.0.0",
@@ -37,6 +39,7 @@
   "devDependencies": {
     "chai": "^4.3.4",
     "mocha": "^9.1.3",
-    "sinon": "^12.0.1"
+    "sinon": "^12.0.1",
+    "typescript": "^4.5.2"
   }
 }

test/types.ts

@@ -0,0 +1,79 @@
+/*
+ * Verify type declarations found in index.d.ts by running:
+ * $ npx tsc --noEmit test/types.ts
+ */
+import fsm from '../index.js';
+
+// @ts-expect-error fsm expects 2 arguments (0 provided)
+const invalid1 = fsm();
+// @ts-expect-error fsm expects 2 arguments (1 provided)
+const invalid2 = fsm('foo');
+// @ts-expect-error fsm expects string or symbol for initial state (null provided)
+const invalid3 = fsm(null, {});
+// @ts-expect-error fsm expects string or symbol for initial state (number provided)
+const invalid4 = fsm(1, {});
+// @ts-expect-error fsm expects object for states (string provided)
+const invalid5 = fsm('foo', 'bar');
+// @ts-expect-error fsm expects initial state to match a defined state or fallback
+const invalid6 = fsm('foo', {});
+
+const invalid7 = fsm('foo', {
+  foo: {
+    // @ts-expect-error state expects action to be string or function (object provided)
+    bar: {},
+    // @ts-expect-error state expects action to be string or function (number provided)
+    baz: 1,
+    // @ts-expect-error state expects lifecycle action to be function (string provided)
+    _enter: 'bar'
+  }
+});
+
+// A simple, valid state machine
+const valid1 = fsm('off', {
+  off: {
+    toggle: 'on'
+  },
+  on: {
+    toggle() {
+      return 'off';
+    }
+  }
+});
+
+// @ts-expect-error subscribe expects callback
+valid1.subscribe();
+const unsub = valid1.subscribe(() => {});
+// @ts-expect-error unsubscribe expects no argumernts
+unsub('foo');
+unsub();
+
+// @ts-expect-error state machine expects valid event invocation
+valid1.noSuchAction();
+
+// @ts-expect-error toggle expects no arguments (1 provided)
+valid1.toggle(1);
+valid1.toggle();
+
+// A state machine with fallback state (any initial state permitted)
+const valid2 = fsm('initial', {
+  '*': {
+    foo: () => {}
+  },
+});
+valid2.foo();
+
+// A state machine with overloaded action signatures
+const valid3 = fsm('initial', {
+  '*': {
+    overloaded(one) {}
+  },
+  'foo': {
+    overloaded(one, two) {}
+  }
+});
+// @ts-expect-error overloaded expects 1 or 2 args (0 provided)
+valid3.overloaded();
+valid3.overloaded(1);
+valid3.overloaded(1, 2);
+// @ts-expect-error overloaded expects 1 or 2 args (3 provided)
+valid3.overloaded(1, 2, 3);

test.js → test/units.js

@@ -1,6 +1,6 @@
 import { assert } from 'chai';
 import sinon from 'sinon';
-import fsm from './index.js';
+import fsm from '../index.js';
 
 sinon.assert.expose(assert, { prefix: '' });