types(shared): improve capitalize types

packages/shared/src/index.ts

@@ -52,7 +52,8 @@ export const isMap = (val: unknown): val is Map<any, any> =>
 export const isSet = (val: unknown): val is Set<any> =>
   toTypeString(val) === '[object Set]'
 
-export const isDate = (val: unknown): val is Date => toTypeString(val) === '[object Date]'
+export const isDate = (val: unknown): val is Date =>
+  toTypeString(val) === '[object Date]'
 export const isFunction = (val: unknown): val is Function =>
   typeof val === 'function'
 export const isString = (val: unknown): val is string => typeof val === 'string'
@@ -99,7 +100,7 @@ const cacheStringFunction = <T extends (str: string) => string>(fn: T): T => {
   return ((str: string) => {
     const hit = cache[str]
     return hit || (cache[str] = fn(str))
-  }) as any
+  }) as T
 }
 
 const camelizeRE = /-(\w)/g
@@ -121,16 +122,17 @@ export const hyphenate = cacheStringFunction((str: string) =>
 /**
  * @private
  */
-export const capitalize = cacheStringFunction(
-  (str: string) => str.charAt(0).toUpperCase() + str.slice(1)
-)
+export const capitalize = cacheStringFunction(<T extends string>(str: T) => {
+  return (str.charAt(0).toUpperCase() + str.slice(1)) as Capitalize<T>
+})
 
 /**
  * @private
  */
-export const toHandlerKey = cacheStringFunction((str: string) =>
-  str ? `on${capitalize(str)}` : ``
-)
+export const toHandlerKey = cacheStringFunction(<T extends string>(str: T) => {
+  const s = str ? `on${capitalize(str)}` : ``
+  return s as T extends '' ? '' : `on${Capitalize<T>}`
+})
 
 // compare whether a value has changed, accounting for NaN.
 export const hasChanged = (value: any, oldValue: any): boolean =>