feat: use discriminated union in useAsync

Author: xiaoxiangmoe

docs/useAsync.md

@@ -17,13 +17,15 @@ const fn = () => new Promise((resolve) => {
 });
 
 const Demo = () => {
-  const {loading, value, error} = useAsync(fn);
+  const state = useAsync(fn);
 
   return (
     <div>
-      {loading
-        ? <div>Loading...</div>
-        : <div>Value: {value}</div>
+      {state.loading?
+        <div>Loading...</div>
+        : state.error?
+        <div>Error...</div>
+        : <div>Value: {state.value}</div>
       }
     </div>
   );

package.json

@@ -43,7 +43,7 @@
     "@storybook/react": "^3.4.11",
     "react": "next",
     "react-dom": "next",
-    "typescript": "^3.1.3",
+    "typescript": "^3.2.2",
     "ts-node": "^7.0.1",
     "ts-loader": "3",
     "babel-core": "^6.26.3",

src/useAsync.ts

@@ -1,10 +1,18 @@
 import {useState, useEffect, useCallback} from 'react';
 
-export interface AsyncState<T> {
-  loading: boolean;
-  error?: Error | any;
-  value?: T;
+export type AsyncState<T> =
+| {
+  loading: true;
 }
+| {
+  loading: false;
+  error: Error;
+}
+| {
+  loading: false;
+  error?: undefined;
+  value: T;
+};
 
 const useAsync = <T>(fn: () => Promise<T>, args?) => {
   const [state, set] = useState<AsyncState<T>>({