check for an invalid id with a repeating reserved character

src/index.ts

@@ -244,7 +244,13 @@ export default class Sqids {
 			const chunks = id.split(separator);
 			if (chunks.length) {
 				// decode the number without using the `separator` character
+				// but also check that ID can be decoded (eg: does not contain any non-alphabet characters)
 				const alphabetWithoutSeparator = alphabet.slice(0, -1);
+				for (const c of chunks[0]) {
+					if (!alphabetWithoutSeparator.includes(c)) {
+						return [];
+					}
+				}
 				ret.push(this.toNumber(chunks[0], alphabetWithoutSeparator));
 
 				// if this ID has multiple numbers, shuffle the alphabet because that's what encoding function did

tests/encoding.test.ts

@@ -113,6 +113,11 @@ test('decoding an ID with an invalid character', () => {
 	expect.soft(sqids.decode('*')).toEqual([]);
 });
 
+test('decoding an invalid ID with a repeating reserved character', () => {
+	const sqids = new Sqids();
+	expect.soft(sqids.decode('fff')).toEqual([]);
+});
+
 test.fails('encode out-of-range numbers', () => {
 	const sqids = new Sqids();
 	expect(sqids.encode([sqids.minValue() - 1])).rejects;