feat(bind): add support for undefined

docs/pages/nullable/bind.mdx

@@ -11,13 +11,14 @@ import { bind } from '@fullstacksjs/toolbox';
 ### Signature
 
 ```typescript copy
-function bind<T, U>(x: Nullable<T>, ...fns: Function[]): U {}
+function bind<T, U>(x: Nullable<T>, ...fns: Function[]): Nullable<U> {}
 ```
 
 ### Examples
 
 ```typescript copy
-bind(null,f)           // null
+bind(undefined, f)     // undefined
+bind(null, f)          // null
 bind(1, f)             // f(1)
 bind(2, f, g, h)       // h(g(f(2)))
 bind(null, f, g, h, j) // null

src/nullable/bind.spec.ts

@@ -31,6 +31,10 @@ describe('bind', () => {
     expect(bind(null, f, g, h, j, k)).eq(null);
   });
 
+  it('should return undefined if first arg is undefined', () => {
+    expect(bind(undefined, f, g, h, j, k)).eq(undefined);
+  });
+
   it('should return null if any composed function returned null', () => {
     expect(bind('normal', f, g, getNull, h, j)).eq(null);
   });

src/nullable/bind.ts

@@ -12,12 +12,13 @@ import type { Nullable } from '../types';
  *
  * @example
  *
+ * bind(undefined ,f)     // undefined
  * bind(null,f)           // null
  * bind(1, f)             // f(1)
  * bind(2, f, g, h)       // h(g(f(2)))
  * bind(null, f, g, h, j) // null
  */
-export function bind(x: null, ...fns: any[]): null;
+export function bind(x: Nullable, ...fns: any[]): Nullable;
 export function bind<A, B>(
   x: Nullable<A>,
   f: (x: A) => Nullable<B>,
@@ -58,5 +59,5 @@ export function bind<A, B, C, D, E, F, G>(
   l: (x: F) => Nullable<G>,
 ): Nullable<G>;
 export function bind<A>(x: Nullable<A>, ...fns: any[]) {
-  return fns.reduce((v, f) => (v === null ? null : f(v)), x);
+  return fns.reduce((v, f) => (v == null ? v : f(v)), x);
 }