Update flow.md

Author: vvscode

topics/flow.md

@@ -109,3 +109,16 @@ serialProcess([1,2,3,4,5], (el, index, list, done) => {
  }, el*100);
 }).then((list) => console.log(list)); // [1,4,9,16,25]
 ```
+in callback style
+
+```javascript
+serialProcess([1,2,3,4,5], (el, index, list, done) => {
+ console.log(`${el} start`);
+ setTimeout(() => {
+ console.log(`${el} end`);
+ done(el*el);
+ }, el*100);
+},
+(list) => console.log(list)
+); // [1,4,9,16,25]
+```