docs: added typescript documentation to plugins.md (#1796)

docs/guide/extending-vtu/plugins.md

@@ -6,9 +6,9 @@ functionality.
 
 Some use cases for plugins:
 
-1.  Aliasing existing public methods
-1.  Attaching matchers to the Wrapper instance
-1.  Attaching functionality to the Wrapper
+1. Aliasing existing public methods
+2. Attaching matchers to the Wrapper instance
+3. Attaching functionality to the Wrapper
 
 ## Wrapper Plugin
 
@@ -168,6 +168,19 @@ const wrapper = mount(Component, {
 })
 ```
 
+## Using the plugin with TypeScript
+
+To use your custom wrapper plugin with [TypeScript](https://www.typescriptlang.org/) you have to declare your custom wrapper function. Therefore, add a file named `vue-test-utils.d.ts` with the following content:
+```typescript
+import { DOMWrapper } from '@vue/test-utils';
+
+declare module '@vue/test-utils' {
+  export class VueWrapper {
+    findByTestId(testId: string): DOMWrapper[];
+  }
+}
+```
+
 ## Featuring Your Plugin
 
 If you're missing functionality, consider writing a plugin to extend Vue Test

src/baseWrapper.ts

@@ -41,7 +41,7 @@ export default abstract class BaseWrapper<ElementType extends Node>
     return this.wrapperElement
   }
 
-  constructor(element: ElementType) {
+  protected constructor(element: ElementType) {
     this.wrapperElement = element
   }
 

src/vueWrapper.ts

@@ -31,10 +31,12 @@ export class VueWrapper<
     $emit: (event: any, ...args: any[]) => void
   } & ComponentCustomProperties = ComponentPublicInstance
 > extends BaseWrapper<Node> {
-  private componentVM: T
-  private rootVM: ComponentPublicInstance | undefined | null
-  private __app: App | null
-  private __setProps: ((props: Record<string, unknown>) => void) | undefined
+  private readonly componentVM: T
+  private readonly rootVM: ComponentPublicInstance | undefined | null
+  private readonly __app: App | null
+  private readonly __setProps:
+    | ((props: Record<string, unknown>) => void)
+    | undefined
   private cleanUpCallbacks: Array<() => void> = []
 
   constructor(