Upgrade Typescript and types

Author: Mark

package-lock.json

@@ -72,7 +72,7 @@
     "sinon": "^9.0.1",
     "tslint": "^6.1.1",
     "tslint-config-prettier": "^1.15.0",
-    "typescript": "^3.2.2",
+    "typescript": "^3.9.6",
     "uglify-es": "^3.3.9"
   }
 }

package.json

@@ -58,7 +58,10 @@ function getTransitionDuration(el: HTMLElement): number {
      */
     const getCssComputedProperty = (prop: string): string => {
         const style = window.getComputedStyle(el);
-        return style.getPropertyValue(prop) || el.style[getJsPropName(prop)];
+        return (
+            style.getPropertyValue(prop) ||
+            (el.style[getJsPropName(prop)] as string)
+        );
     };
 
     const delayProp = getCssComputedProperty('transition-delay') || '0ms';