fix(types): allow string for watch handlers in options (#10396)

* Allow watch handler to be string as it is supported

* Move string type to WatchHandler itself

* Removed string type as it moved to WatchHandler

Author: OrkhanAlikhanov

types/options.d.ts

@@ -76,7 +76,7 @@ export interface ComponentOptions<
   propsData?: object;
   computed?: Accessors<Computed>;
   methods?: Methods;
-  watch?: Record<string, WatchOptionsWithHandler<any> | WatchHandler<any> | string>;
+  watch?: Record<string, WatchOptionsWithHandler<any> | WatchHandler<any>>;
 
   el?: Element | string;
   template?: string;
@@ -169,7 +169,7 @@ export interface ComputedOptions<T> {
   cache?: boolean;
 }
 
-export type WatchHandler<T> = (val: T, oldVal: T) => void;
+export type WatchHandler<T> = string | ((val: T, oldVal: T) => void);
 
 export interface WatchOptions {
   deep?: boolean;

types/test/options-test.ts

@@ -168,6 +168,10 @@ Vue.component('component', {
         this.a = val
       },
       deep: true
+    },
+    d: {
+      handler: 'someMethod',
+      immediate: true
     }
   },
   el: "#app",