Merge pull request coinbase#210 from blair/promise-each-series-templa…

…te-parameter-for-return-type

eachSeries: be more specific on the return type.

src/utils/promises.ts

@@ -21,12 +21,12 @@ export function delay(time: number): Promise<void> {
 /**
  * Apply each argument in arr to iteratorFn, waiting for the promise to resolve before continuing
  */
-export function eachSeries<T>(arr: T[], iteratorFn: (arg: T) => Promise<any>) {
+export function eachSeries<T, U>(arr: T[], iteratorFn: (arg: T) => Promise<U>): Promise<null | U> {
     return arr.reduce((prev, item) => {
         return prev.then(() => {
             return iteratorFn(item);
         });
-    }, Promise.resolve());
+    }, Promise.resolve(null));
 }
 
 /**