refactor: improve time units referencing

lib/time.js

@@ -42,14 +42,15 @@ const ALIASES = {
 	fri: 5,
 	sat: 6
 };
-const TIME_UNITS = [
-	'second',
-	'minute',
-	'hour',
-	'dayOfMonth',
-	'month',
-	'dayOfWeek'
-];
+const TIME_UNITS_MAP = {
+	SECOND: 'second',
+	MINUTE: 'minute',
+	HOUR: 'hour',
+	DAY_OF_MONTH: 'dayOfMonth',
+	MONTH: 'month',
+	DAY_OF_WEEK: 'dayOfWeek'
+};
+const TIME_UNITS = Object.values(TIME_UNITS_MAP);
 const TIME_UNITS_LEN = TIME_UNITS.length;
 const PRESETS = {
 	'@yearly': '0 0 0 1 1 *',
@@ -672,7 +673,10 @@ function CronTime(luxon) {
 		_hasAll: function (type) {
 			const constraints = CONSTRAINTS[TIME_UNITS.indexOf(type)];
 			const low = constraints[0];
-			const high = type === 'dayOfWeek' ? constraints[1] - 1 : constraints[1];
+			const high =
+				type === TIME_UNITS_MAP.DAY_OF_WEEK
+					? constraints[1] - 1
+					: constraints[1];
 
 			for (let i = low, n = high; i < n; i++) {
 				if (!(i in this[type])) {