Switched usage of Array.prototype.findIndex to indexOf.

README.md

@@ -246,5 +246,3 @@ controlled manually thereafter.
 | Chrome        | Desktop   | latest  |
 | Firefox       | Desktop   | latest  |
 | Safari        | OSX       | latest  |
-
->Note: This library does not have polyfill support for `Array.prototype.findIndex`, so if you require support **IE 11** you will need to add polyfill in your build or manually add it.

src/helpers/AccordionStore.ts

@@ -78,11 +78,7 @@ export default class AccordionStore {
     };
 
     public readonly isItemExpanded = (uuid: UUID): boolean => {
-        return (
-            this.expanded.findIndex(
-                (expandedUuid: UUID) => expandedUuid === uuid,
-            ) !== -1
-        );
+        return this.expanded.indexOf(uuid) !== -1;
     };
 
     public readonly getPanelAttributes = (