chore: update isNull and isUndefined

Author: hunghg255

docs/reference/predicate/isNull.md

@@ -10,7 +10,7 @@ This function can also serve as a type predicate in TypeScript, narrowing the ty
 ## Signature
 
 ```typescript
-function isNull(x: unknown): x is null;
+function isNull(val: any): val is null
 ```
 
 ### Parameters

docs/reference/predicate/isUndefined.md

@@ -10,12 +10,12 @@ This function can also serve as a type predicate in TypeScript, narrowing the ty
 ## Signature
 
 ```typescript
-function isUndefined(x: unknown): x is undefined;
+function isUndefined(val: any): val is undefined
 ```
 
 ### Parameters
 
-- `x` (`unknown`): The value to test if it is `undefined`.
+- `x` (`any`): The value to test if it is `undefined`.
 
 ### Returns
 

src/predicate/isNull.ts

@@ -1,3 +1,5 @@
+import { toString } from './_internal/toString';
+
 /**
  * Checks if the given value is null.
  *
@@ -18,6 +20,7 @@
  * console.log(isNull(value2)); // false
  * console.log(isNull(value3)); // false
  */
-export function isNull(x: unknown): x is null {
-  return x === null;
+
+export function isNull(val: any): val is null {
+  return toString(val) === '[object Null]';
 }

src/predicate/isUndefined.ts

@@ -1,3 +1,5 @@
+import { toString } from './_internal/toString';
+
 /**
  * Checks if the given value is undefined.
  *
@@ -18,6 +20,7 @@
  * console.log(isUndefined(value2)); // false
  * console.log(isUndefined(value3)); // false
  */
-export function isUndefined(x: unknown): x is undefined {
-  return x === undefined;
+
+export function isUndefined(val: any): val is undefined {
+  return toString(val) === '[object Undefined]';
 }