feat: add a fallback method on start (#321)

* feat: add a fallback method on start

* docs: add onStartCallback doc

* bundle: build js

Author: cherrol

README.md

@@ -69,6 +69,7 @@ interface CountUpOptions {
   scrollSpyDelay?: number; // delay (ms) after target comes into view
   scrollSpyOnce?: boolean; // run only once
   onCompleteCallback?: () => any; // gets called when animation completes
+  onStartCallback?: () => any; // gets called when animation start
   plugin?: CountUpPlugin; // for alternate animations
 }
 ```
@@ -89,7 +90,13 @@ Pass options:
 const countUp = new CountUp('targetId', 5234, options);
 ```
 
-with optional callback:
+with optional start callback:
+
+```js
+const countUp = new CountUp('targetId', 5234, { onStartCallback: someMethod });
+```
+
+with optional complete callback:
 
 ```js
 const countUp = new CountUp('targetId', 5234, { onCompleteCallback: someMethod });

dist/countUp.d.ts

@@ -18,6 +18,7 @@ export interface CountUpOptions {
     scrollSpyDelay?: number;
     scrollSpyOnce?: boolean;
     onCompleteCallback?: () => any;
+    onStartCallback?: () => any;
     plugin?: CountUpPlugin;
 }
 export declare interface CountUpPlugin {

dist/countUp.js

@@ -206,6 +206,9 @@ var CountUp = /** @class */ (function () {
         if (this.error) {
             return;
         }
+        if (this.options.onStartCallback) {
+            this.options.onStartCallback();
+        }
         if (callback) {
             this.options.onCompleteCallback = callback;
         }