perf: Avoid redundant offset/abbr checks in tz.guess (non-Intl fallback)

Author: gilmoreorless

moment-timezone.js

@@ -309,17 +309,21 @@
 	function userOffsets() {
 		var startYear = new Date().getFullYear() - 2,
 			last = new OffsetAt(new Date(startYear, 0, 1)),
+			lastOffset = last.offset,
 			offsets = [last],
-			change, next, i;
+			change, next, nextOffset, i;
 
 		for (i = 1; i < 48; i++) {
-			next = new OffsetAt(new Date(startYear, i, 1));
-			if (next.offset !== last.offset) {
+			nextOffset = new Date(startYear, i, 1).getTimezoneOffset();
+			if (nextOffset !== lastOffset) {
+				// Create OffsetAt here to avoid unnecessary abbr parsing before checking offsets
+				next = new OffsetAt(new Date(startYear, i, 1));
 				change = findChange(last, next);
 				offsets.push(change);
 				offsets.push(new OffsetAt(new Date(change.at + 6e4)));
+				last = next;
+				lastOffset = nextOffset;
 			}
-			last = next;
 		}
 
 		for (i = 0; i < 4; i++) {
@@ -357,15 +361,21 @@
 		var offsetsLength = offsets.length,
 			filteredGuesses = {},
 			out = [],
-			i, j, guessesOffset;
+			checkedOffsets = {},
+			i, j, offset, guessesOffset;
 
 		for (i = 0; i < offsetsLength; i++) {
-			guessesOffset = guesses[offsets[i].offset] || {};
+			offset = offsets[i].offset;
+			if (checkedOffsets.hasOwnProperty(offset)) {
+				continue;
+			}
+			guessesOffset = guesses[offset] || {};
 			for (j in guessesOffset) {
 				if (guessesOffset.hasOwnProperty(j)) {
 					filteredGuesses[j] = true;
 				}
 			}
+			checkedOffsets[offset] = true;
 		}
 
 		for (i in filteredGuesses) {