Add comments to isRegionalIndicator

Signed-off-by: Robin Townsend <robin@robin.town>

Author: robintown

src/emoji.ts

@@ -35,8 +35,15 @@ export const EMOTICON_TO_EMOJI = new Map<string, IEmoji>();
 
 export const getEmojiFromUnicode = unicode => UNICODE_TO_EMOJI.get(stripVariation(unicode));
 
-const isRegionalIndicator = (x: string): boolean =>
- Array.from(x).length === 1 && x >= '\u{1f1e6}' && x <= '\u{1f1ff}';
+const isRegionalIndicator = (x: string): boolean => {
+ // First verify that the string is a single character. We use Array.from
+ // to make sure we count by characters, not UTF-8 code units.
+ return Array.from(x).length === 1 &&
+ // Next verify that the character is within the code point range for
+ // regional indicators.
+ // http://unicode.org/charts/PDF/Unicode-6.0/U60-1F100.pdf
+ x >= '\u{1f1e6}' && x <= '\u{1f1ff}';
+};
 
 const EMOJIBASE_GROUP_ID_TO_CATEGORY = [
  "people", // smileys