RFC: Format snapshot changes ourselves

This misappropriates Jest's reporter functionality so we can plug in to
test file execution and re-format files that have updated snapshots.

Alternative to #1220.

.changeset/nine-keys-accept.md

@@ -2,7 +2,7 @@
 'skuba': patch
 ---
 
-test: Disable Prettier snapshot formatting
+test: Fix Prettier snapshot formatting
 
 Jest is not yet compatible with Prettier 3, causing snapshot updates to fail with the following error:
 
@@ -11,12 +11,12 @@ TypeError: prettier.resolveConfig.sync is not a function
     at runPrettier (node_modules/jest-snapshot/build/InlineSnapshots.js:308:30)
 ```
 
-We have disabled Prettier snapshot formatting in our Jest preset as a temporary workaround. If you do not use the preset, you can manually modify your `jest.config.ts` like so:
+Our [Jest preset](https://seek-oss.github.io/skuba/docs/development-api/jest.html#mergepreset) now implements custom Prettier formatting as a temporary workaround until [jestjs/jest#14305](https://github.com/jestjs/jest/issues/14305) is resolved.
+
+If you do not use our preset, you can temporarily disable Prettier snapshot formatting in your `jest.config.ts` then manually run `skuba format` after updating snapshots:
 
 ```diff
 export default {
 + prettierPath: null,
 }
 ```
-
-You may have to run `skuba format` manually after updating snapshots until [jestjs/jest#14305](https://github.com/jestjs/jest/issues/14305) is resolved.

jest-preset.js

@@ -21,14 +21,18 @@ module.exports = {
     '!<rootDir>/jest.*.ts',
   ],
   coverageDirectory: 'coverage',
-  reporters: ['default', require.resolve('./lib/cli/test/reporters/github')],
+  // jestjs/jest#14305
+  prettierPath: null,
+  reporters: [
+    'default',
+    require.resolve('./lib/cli/test/reporters/github'),
+    require.resolve('./lib/cli/test/reporters/prettier'),
+  ],
   testEnvironment: 'node',
   testPathIgnorePatterns: [
     '/node_modules.*/',
     '<rootDir>/(coverage|dist|lib|tmp).*/',
   ],
-  // jestjs/jest#14305
-  prettierPath: null,
   watchPlugins: [
     require.resolve('jest-watch-typeahead/filename'),
     require.resolve('jest-watch-typeahead/testname'),

src/cli/adapter/prettier.ts

@@ -83,10 +83,10 @@ interface Result {
   unparsed: string[];
 }
 
-const formatOrLintFile = async (
+export const formatOrLintFile = async (
   { data, filepath, options }: File,
   mode: 'format' | 'lint',
-  result: Result,
+  result: Result | null,
 ): Promise<string | undefined> => {
   if (mode === 'lint') {
     let ok: boolean;
@@ -95,12 +95,12 @@ const formatOrLintFile = async (
         (await check(data, options)) &&
         (await isPackageJsonOk({ data, filepath }));
     } catch (err) {
-      result.errored.push({ err, filepath });
+      result?.errored.push({ err, filepath });
       return;
     }
 
     if (!ok) {
-      result.errored.push({ filepath });
+      result?.errored.push({ filepath });
     }
 
     return;
@@ -110,7 +110,7 @@ const formatOrLintFile = async (
   try {
     formatted = await format(data, options);
   } catch (err) {
-    result.errored.push({ err, filepath });
+    result?.errored.push({ err, filepath });
     return;
   }
 
@@ -130,7 +130,7 @@ const formatOrLintFile = async (
     return;
   }
 
-  result.touched.push(filepath);
+  result?.touched.push(filepath);
   return formatted;
 };
 

src/cli/test/reporters/prettier/index.ts

@@ -0,0 +1,45 @@
+import type { Reporter } from '@jest/reporters';
+import fs from 'fs-extra';
+import { resolveConfig } from 'prettier';
+
+import { formatOrLintFile } from '../../../../cli/adapter/prettier';
+
+export default class SnapshotPrettifier
+  implements Pick<Reporter, 'onTestFileResult'>
+{
+  async onTestFileResult(
+    ...[test, testResult]: Parameters<NonNullable<Reporter['onTestFileResult']>>
+  ): Promise<void> {
+    if (!testResult.snapshot.added && !testResult.snapshot.updated) {
+      return;
+    }
+
+    const filepath = test.path;
+
+    // This is a best-effort workaround to automatically format code.
+    // Don't pollute console output if it fails for whatever reason.
+    try {
+      const [config, data] = await Promise.all([
+        resolveConfig(filepath),
+        fs.promises.readFile(filepath, 'utf-8'),
+      ]);
+
+      const formatted = await formatOrLintFile(
+        {
+          data,
+          filepath,
+          options: {
+            ...config,
+            filepath,
+          },
+        },
+        'format',
+        null,
+      );
+
+      if (typeof formatted === 'string') {
+        await fs.promises.writeFile(filepath, formatted);
+      }
+    } catch {}
+  }
+}