fix EAN-13 checksum calculation, add unit tests

src/items/ean/ean.service.ts

@@ -9,11 +9,12 @@ export class EANService {
     let sum = id
       .split('')
       .map((d) => parseInt(d, 10))
-      .reduce((s, d, i) => s + d * (i % 2 === 0 ? 3 : 1), 10);
+      .map((d, i) => d * (i % 2 === 0 ? 1 : 3))
+      .reduce((s, d) => s + d, 0);
     while (sum >= 10) {
       sum -= 10;
     }
-    return sum;
+    return 10 - sum;
   }
 
   isValid(ean: string) {
@@ -28,6 +29,7 @@ export class EANService {
     if (!this.isValid(ean)) {
       throw new Error(`Invalid EAN: "${ean}"`);
     }
+    return ean.slice(0, 12);
   }
 
   fromID(id: string) {

src/items/ean/ean.spec.ts

@@ -0,0 +1,9 @@
+import { EANService } from './ean.service';
+
+describe('EANService', () => {
+  const eans = new EANService();
+  test('generates valid EAN-13 checksum', () => {
+    const sum = eans.calcChecksum('400638133393');
+    expect(sum).toBe(1);
+  });
+});