chainAsync update: last function shouldn't get 'next' as argument

Confident that my tests pass, but _30.js won't update.

snippets/chainAsync.md

@@ -6,8 +6,11 @@ Loop through an array of functions containing asynchronous events, calling `next
 
 ```js
 const chainAsync = fns => {
-  let curr = 0;
-  const next = () => fns[curr++](next);
+  let curr = 0, last = fns[fns.length - 1];
+  const next = () => {
+    const fn = fns[curr++]
+    fn(fn === last ? null : next)
+  }
   next();
 };
 ```
@@ -20,6 +23,10 @@ chainAsync([
   },
   next => {
     console.log('1 second');
+    setTimeout(next, 1000);
+  },
+  () => {
+    console.log('2 second');
   }
 ]);
 ```

test/chainAsync.test.js

@@ -5,18 +5,30 @@ test('chainAsync is a Function', () => {
   expect(chainAsync).toBeInstanceOf(Function);
 });
 
+let incrementer = 0
 test('Calls all functions in an array', () => {
+  chainAsync([
+    next => {
+      incrementer += 1
+      next();
+    },
+    next => {
+      incrementer += 1
+      next();
+    },
+    next => {
+      expect(incrementer).toEqual(2);
+    }
+  ]);
+});
+
+test('Last function does not receive "next" argument', () => {
   chainAsync([
     next => {
       next();
     },
     next => {
-      (() => {
-        next();
-      })();
-    },
-    next => {
-      expect(true).toBeTruthy();
+      expect(next).toBe(null);
     }
   ]);
-});
+})