fix: isString should properly guard string type

src/__tests__/advanced.spec.ts

@@ -0,0 +1,68 @@
+import { describe, test, expect, vi } from 'vitest'
+import { createComposableFromMixin } from '../createComposable'
+import { defineMixin } from '../defineMixin'
+import { wrapComposable } from './helpers'
+
+describe('advanced', () => {
+  test('nested mixins', async () => {
+    const innerSpy = vi.fn()
+    const outerSpy = vi.fn()
+    const innerMixin = defineMixin({
+      props: {
+        inner: Number,
+      },
+      data: () => ({
+        nestedProperty: 'A',
+        sharedProperty: 'A',
+      }),
+      created() {
+        innerSpy()
+      },
+      mounted() {
+        innerSpy()
+      },
+    })
+    const outerMixin = defineMixin({
+      mixins: [innerMixin],
+      props: {
+        outer: String,
+      },
+      data: () => ({
+        msg: 'Hello World',
+        sharedProperty: 'B',
+      }),
+      computed: {
+        getNestedProperty(): string {
+          return this.nestedProperty
+        },
+      },
+      mounted() {
+        outerSpy()
+      },
+    })
+    const innerComposable = createComposableFromMixin(innerMixin)
+
+    const outerComposable = createComposableFromMixin(outerMixin)
+    const wrapper = wrapComposable(
+      outerComposable,
+      {
+        props: {
+          outer: 'Hello',
+          inner: 10,
+        },
+      },
+      {
+        props: outerComposable.props,
+      }
+    )
+    // Props
+    expect(wrapper.vm.outer).toBe('Hello')
+    expect(wrapper.vm.inner).toBe(10)
+    // Data & computed
+    expect(wrapper.vm.sharedProperty).toBe('B')
+    expect(wrapper.vm.getNestedProperty).toBe('A')
+    // Lifecycle Hooks
+    expect(innerSpy).toHaveBeenCalledTimes(2)
+    expect(outerSpy).toHaveBeenCalledTimes(1)
+  })
+})

src/utils.ts

@@ -5,7 +5,7 @@ export const isFunction = (val: unknown): val is Function =>
 export const isArray = Array.isArray
 export const isObject = (val: unknown): val is Record<any, any> =>
   val !== null && typeof val === 'object'
-export const isString = (val: unknown): val is String => typeof val === 'string'
+export const isString = (val: unknown): val is string => typeof val === 'string'
 
 /**
  * @legal