fix(npm): add trailing newline when composing lockfiles (#14753)

lib/modules/manager/npm/__snapshots__/utils.spec.ts.snap

@@ -13,5 +13,6 @@ exports[`modules/manager/npm/utils composeLockFile composes lockfile string out
   },
   \\"requires\\": true,
   \\"version\\": \\"1.0.0\\"
-}"
+}
+"
 `;

lib/modules/manager/npm/post-update/__snapshots__/npm.spec.ts.snap

@@ -176,7 +176,8 @@ exports[`modules/manager/npm/post-update/npm performs lock file updates retainin
       \\"integrity\\": \\"sha512-R0XvVJ9WusLiqTCEiGCmICCMplcCkIwwR11mOSD9CR5u+IXYdiseeEuXCVAjS54zqwkLcPNnmU4OeJ6tUrWhDw==\\"
     }
   }
-}"
+}
+"
 `;
 
 exports[`modules/manager/npm/post-update/npm performs lock file updates retaining the package.json counterparts 2`] = `

lib/modules/manager/npm/utils.spec.ts

@@ -53,5 +53,12 @@ describe('modules/manager/npm/utils', () => {
       const lockFileComposed = composeLockFile(lockFile, '  ');
       expect(lockFileComposed).toMatchSnapshot();
     });
+
+    it('adds trailing newline to match npms behaviour and avoid diffs', () => {
+      const lockFile = loadFixture('lockfile-parsing/package-lock.json');
+      const { detectedIndent, lockFileParsed } = parseLockFile(lockFile);
+      const lockFileComposed = composeLockFile(lockFileParsed, detectedIndent);
+      expect(lockFileComposed).toBe(lockFile);
+    });
   });
 });

lib/modules/manager/npm/utils.ts

@@ -16,5 +16,5 @@ export function parseLockFile(lockFile: string): ParseLockFileResult {
 }
 
 export function composeLockFile(lockFile: LockFile, indent: string): string {
-  return JSON.stringify(lockFile, null, indent);
+  return JSON.stringify(lockFile, null, indent) + '\n';
 }