Feat: Support custom clone operation

README.md

@@ -71,6 +71,11 @@ const ignoreFormat = createFormat({
     const unique = (array) => Array.from(new Set() < T[number] > array).sort()
     await fs.writeFile(resolvedPath, unique(expected).join('\n'), 'utf8')
   },
+  // Optional: define a 'clone' function to control how the result of 'read' is cloned before being passed to 'update'
+  // Defaults to `structuredClone`[1] if available, otherwise `v8.deserialize(v8.serialize(obj))`.
+  clone(actual) {
+    return myCustomClone(actual)
+  },
 })
 
 export default async (_workspaceDir) => {

src/index.ts

@@ -57,14 +57,14 @@ type UpdateError =
 
 export async function performUpdates<
   FileNameWithOptions extends string,
-  UserDefinedFormatPlugins extends BaseFormatPlugins
+  UserDefinedFormatPlugins extends BaseFormatPlugins,
 >(
   workspaceDir: string,
   updateParam:
     | UpdateOptionsLegacy<FileNameWithOptions>
     | UpdateOptions<FileNameWithOptions>
     | UpdateOptionsWithFormats<FileNameWithOptions, UserDefinedFormatPlugins>,
-  opts?: { test?: boolean }
+  opts?: { test?: boolean },
 ): Promise<null | UpdateError[]> {
   const update = 'files' in updateParam ? updateParam : { files: updateParam }
   let pkgs = await findWorkspacePackagesNoCheck(workspaceDir)
@@ -88,7 +88,11 @@ export async function performUpdates<
           _writeProjectManifest: writeProjectManifest,
         }
         const actual = (await fileExists(resolvedPath)) ? await formatPlugin.read(formatHandlerOptions) : null
-        const expected = await formatPlugin.update(clone(actual), updateFile as any, formatHandlerOptions)
+        const expected = await formatPlugin.update(
+          formatPlugin.clone ? formatPlugin.clone(actual, formatHandlerOptions) : clone(actual),
+          updateFile as any,
+          formatHandlerOptions,
+        )
         const equal =
           (actual == null && expected == null) ||
           (actual != null && expected != null && (await formatPlugin.equal(expected, actual, formatHandlerOptions)))
@@ -107,8 +111,8 @@ export async function performUpdates<
 
         errors.push({ actual, expected, path: resolvedPath })
       } catch (error) {
-        const errorMessage = `Error while processing ${resolvedPath}: ${error.message}`;
-        errors.push({ exception: errorMessage });
+        const errorMessage = `Error while processing ${resolvedPath}: ${error.message}`
+        errors.push({ exception: errorMessage })
       }
     }
   }
@@ -120,7 +124,7 @@ function printJsonDiff(actual: unknown, expected: unknown, out: NodeJS.WriteStre
   printDiff(
     typeof actual !== 'string' ? JSON.stringify(actual, null, 2) : actual,
     typeof expected !== 'string' ? JSON.stringify(expected, null, 2) : expected,
-    out
+    out,
   )
 }
 
@@ -141,7 +145,7 @@ function parseFileKey(fileKey: string, formatPlugins: Record<string, FormatPlugi
 
     if (!formatPlugin) {
       throw new Error(
-        `Configuration error: there is no format plugin for fileKey "${fileKey}" with explicit format specifier "${extension}"`
+        `Configuration error: there is no format plugin for fileKey "${fileKey}" with explicit format specifier "${extension}"`,
       )
     }
 
@@ -156,8 +160,8 @@ function parseFileKey(fileKey: string, formatPlugins: Record<string, FormatPlugi
   if (!extension) {
     throw new Error(
       `Configuration error: there is no format plugin for fileKey "${fileKey}", supported extensions are ${Object.keys(
-        formatPlugins
-      )}`
+        formatPlugins,
+      )}`,
     )
   }
 

src/updater/formatPlugin.ts

@@ -16,14 +16,21 @@ export interface FormatPlugin<Content> {
   update(
     actual: Content | null,
     updater: Updater<Content>,
-    options: FormatPluginFnOptions
+    options: FormatPluginFnOptions,
   ): PromiseOrValue<Content | null>
 
   /** Called only if check for equality is required (`actual != null` & `expected != null`) */
   equal(expected: Content, actual: Content, options: FormatPluginFnOptions): PromiseOrValue<boolean>
 
   /** Called only if write is required (`--test` isn't specified, `expected != null` and `expected` is not equal to `actual`) */
   write(expected: Content, options: FormatPluginFnOptions): PromiseOrValue<void>
+
+  /**
+   * Used to clone the object returned by `read` before passing it to `update`.
+   * Defaults to `structuredClone`[1] if available, otherwise `v8.deserialize(v8.serialize(obj))`.
+   * [1]: https://developer.mozilla.org/en-US/docs/web/api/structuredclone
+   */
+  clone?(value: Content, options: FormatPluginFnOptions): Content
 }
 
 export interface FormatPluginFnOptions {