diff --git a/types/webpack-manifest-plugin/index.d.ts b/types/webpack-manifest-plugin/index.d.ts
index c0d9067a1c..59222d838e 100644
--- a/types/webpack-manifest-plugin/index.d.ts
+++ b/types/webpack-manifest-plugin/index.d.ts
@@ -77,7 +77,7 @@ declare namespace WebpackManifestPlugin {
         /**
          * Sort files before they are passed to generate.
          */
-        sort?: (file: FileDescriptor) => number;
+        sort?: (a: FileDescriptor, b: FileDescriptor) => number;
 
         /**
          * Create the manifest. It can return anything as long as it's serialisable by JSON.stringify.
diff --git a/types/webpack-manifest-plugin/webpack-manifest-plugin-tests.ts b/types/webpack-manifest-plugin/webpack-manifest-plugin-tests.ts
index 3c34f052fa..b52ae5555b 100644
--- a/types/webpack-manifest-plugin/webpack-manifest-plugin-tests.ts
+++ b/types/webpack-manifest-plugin/webpack-manifest-plugin-tests.ts
@@ -14,14 +14,12 @@ const options: WebpackManifestPlugin.Options = {
     map: (file) => {
         if (file.name) {
             file.name = path.join(path.dirname(file.path), file.name);
-            return file;
-        }
-    },
-    reduce: (manifest, { name, path }) => {
-        if (name) {
-            return { ...manifest, [name]: path };
         }
+        return file;
     },
+    sort: (a, b) => a.path.localeCompare(b.path),
+    generate: (seed, files) => files.reduce((manifest, {name, path}) => name ? {...manifest, [name]: path} : manifest, seed),
+    serialize: (manifest) => JSON.stringify(manifest, null, 2)
 };
 
 const c: Configuration = {