Guard TakeLeft Function

Author: sinclairzx81

src/guard/guard.ts

@@ -185,8 +185,8 @@ export function EveryAll<T>(value: T[], offset: number, callback: (value: T, ind
 // ------------------------------------------------------------------
 export type TakeLeftTrue<T> = (left: T, right: T[]) => unknown
 export type TakeLeftFalse = () => unknown
-export function TakeLeft<T>(array: T[], true_: TakeLeftTrue<T>, false_: TakeLeftFalse): unknown {
-  return IsEqual(array.length, 0) ? false_() : true_(array[0], array.slice(1))
+export function TakeLeft<T, True extends TakeLeftTrue<T>, False extends TakeLeftFalse>(array: T[], true_: True, false_: False): ReturnType<True> | ReturnType<False> {
+  return (IsEqual(array.length, 0) ? false_() : true_(array[0], array.slice(1))) as never
 }
 // --------------------------------------------------------------------------
 // Object

test/typebox/runtime/guard/guard.ts

@@ -324,3 +324,25 @@ Test('Should IsClassInstance 15', () => {
   const strObj = new String('abc')
   Assert.IsEqual(Guard.IsClassInstance(strObj), true)
 })
+
+// ------------------------------------------------------------------
+// Guard.TakeLeft
+// ------------------------------------------------------------------
+Test('Should TakeLeft 1', () => {
+  const result: any = Guard.TakeLeft([], (left, right) => ({ left, right }), () => 'empty')
+  Assert.IsEqual(result, 'empty')
+})
+Test('Should TakeLeft 2', () => {
+  const result: string | {
+    left: number
+    right: number[]
+  } = Guard.TakeLeft([1, 2, 3], (left, right) => ({ left, right }), () => 'empty')
+  Assert.IsEqual(result, { left: 1, right: [2, 3] })
+})
+Test('Should TakeLeft 3', () => {
+  const result: string | {
+    left: number
+    right: number[]
+  } = Guard.TakeLeft([42], (left, right) => ({ left, right }), () => 'empty')
+  Assert.IsEqual(result, { left: 42, right: [] })
+})