fix: do not hook assetEmitted when writeToDisk is true (#1)

When `writeToDisk: true`, `setupOutputFileSystem` already configures
webpack to use the native (disk) output file system, so webpack writes
all assets to disk itself.  Calling `setupWriteToDisk` on top of that
tapped `assetEmitted` and wrote every file a second time.

Only invoke `setupWriteToDisk` when `writeToDisk` is a filter function,
where the in-memory fs is still used and selective disk writes are
needed.

Fixes #2270

Author: AdrianBannister

src/index.js

@@ -229,7 +229,7 @@ function wdm(compiler, options = {}) {
 
   setupHooks(context);
 
-  if (options.writeToDisk) {
+  if (typeof options.writeToDisk === "function") {
     setupWriteToDisk(context);
   }
 

test/middleware.test.js

@@ -4056,7 +4056,7 @@ describe.each([
                 compiler.hooks.assetEmitted.taps.filter(
                   (hook) => hook.name === "DevMiddleware",
                 ),
-              ).toHaveLength(1);
+              ).toHaveLength(0);
               expect(fs.existsSync(bundlePath)).toBe(true);
 
               instance.invalidate();
@@ -4068,7 +4068,7 @@ describe.each([
                     compiler.hooks.assetEmitted.taps.filter(
                       (hook) => hook.name === "DevMiddleware",
                     ),
-                  ).toHaveLength(1);
+                  ).toHaveLength(0);
 
                   done();
                 },
@@ -4154,7 +4154,7 @@ describe.each([
               compiler.hooks.assetEmitted.taps.filter(
                 (hook) => hook.name === "DevMiddleware",
               ),
-            ).toHaveLength(1);
+            ).toHaveLength(0);
             expect(fs.existsSync(bundlePath)).toBe(true);
 
             instance.invalidate();
@@ -4166,7 +4166,7 @@ describe.each([
                   compiler.hooks.assetEmitted.taps.filter(
                     (hook) => hook.name === "DevMiddleware",
                   ),
-                ).toHaveLength(1);
+                ).toHaveLength(0);
 
                 done();
               },