diff --git a/src/Calendar.js b/src/Calendar.js
index b7f77e2..b9a7dac 100644
--- a/src/Calendar.js
+++ b/src/Calendar.js
@@ -146,15 +146,16 @@ function Calendar(element, instanceOptions) {
 	// `weekNumberCalculation` setting.
 	t.calculateWeekNumber = function(mom) {
 		var calc = options.weekNumberCalculation;
-		if (calc == 'local') {
+
+		if (typeof calc === 'function') {
+			return calc(mom);
+		}
+		else if (calc === 'local') {
 			return mom.week();
 		}
-		else if (calc.toUpperCase() == 'ISO') {
+		else if (calc.toUpperCase() === 'ISO') {
 			return mom.isoWeek();
 		}
-		else if (typeof calc === 'function') {
-			return calc(mom);
-		}
 	};