chore: add is function helper

src/misc.ts

@@ -1,8 +1,12 @@
 
-export function isObject(val: any) {
+export function isObject(val: any): val is Record<string, unknown> {
     return Object.prototype.toString.call(val) === '[object Object]';
 }
 
+export function isFunction(val: any): val is (...args: any[]) => any {
+    return typeof val === 'function';
+}
+
 export function formDataFromObject(obj: Record<string, any>) {
     const formData = new FormData();
     Object.entries(obj)

tests/misc.spec.ts

@@ -1,4 +1,4 @@
-import { formDataFromObject, isObject, urlSearchParamsFromObject } from '../src/misc';
+import { formDataFromObject, isFunction, isObject, urlSearchParamsFromObject } from '../src/misc';
 
 describe('misc', () => {
     describe('isObject', () => {
@@ -19,6 +19,24 @@ describe('misc', () => {
         });
     });
 
+    describe('isFunction', () => {
+        it('should return thruthy successfully', () => {
+            expect(isFunction(function () { return null; })).toBeTruthy();
+        });
+
+        it('should return falsy', () => {
+            expect(isFunction(1)).toBeFalsy();
+            expect(isFunction(true)).toBeFalsy();
+            expect(isFunction('')).toBeFalsy();
+            expect(isFunction([])).toBeFalsy();
+            expect(isFunction({})).toBeFalsy();
+            expect(isFunction(null)).toBeFalsy();
+            expect(isFunction(undefined)).toBeFalsy();
+            expect(isFunction(NaN)).toBeFalsy();
+            expect(isFunction(Infinity)).toBeFalsy();
+        });
+    });
+
     describe('formDataFromObject', () => {
         it('should convert object to form data successfully', () => {
             const formData = formDataFromObject({