fix: prevent release overflow

src/index.ts

@@ -193,6 +193,12 @@ export class Sema {
 	}
 
 	release(token?: any): void {
+		if (this.waiting.length === 0 && this.free.length >= this.nrTokens) {
+			throw new Error(
+				'Sema#release() was called more times than Sema#acquire()'
+			);
+		}
+
 		this.releaseEmitter.emit('release', this.noTokens ? '1' : token);
 	}
 

test/sema.test.ts

@@ -41,6 +41,24 @@ test('tryAcquire returns undefined', async () => {
 	expect(s.tryAcquire()).toBeUndefined();
 });
 
+test('release throws when called without an acquired token', () => {
+	const s = new Sema(1);
+
+	expect(() => s.release()).toThrow(
+		'Sema#release() was called more times than Sema#acquire()'
+	);
+});
+
+test('release does not increase available tokens beyond the limit', async () => {
+	const s = new Sema(1);
+
+	await s.acquire();
+	s.release();
+
+	expect(s.tryAcquire()).toBeDefined();
+	expect(s.tryAcquire()).toBeUndefined();
+});
+
 test('Pausing works', () => {
 	const pauseFn = jest.fn();
 	const resumeFn = jest.fn();