fix: Preserve trailing newlines when modifying config files

e2e-tests/tests/pnpm-workspace.test.ts

@@ -101,7 +101,8 @@ describe('pnpm workspace', () => {
                 org: "${TEST_ARGS.ORG_SLUG}",
                 project: "${TEST_ARGS.PROJECT_SLUG}"
             }), sveltekit()]
-        });"
+        });
+        "
       `);
   });
 

e2e-tests/tests/sveltekit-tracing.test.ts

@@ -105,7 +105,8 @@ describe('Sveltekit with instrumentation and tracing', () => {
                 org: "${TEST_ARGS.ORG_SLUG}",
                 project: "${TEST_ARGS.PROJECT_SLUG}"
             }), sveltekit()]
-        });"
+        });
+        "
       `);
     });
 

src/react-native/javascript.ts

@@ -11,6 +11,7 @@ import { traceStep } from '../telemetry';
 import { makeCodeSnippet, showCopyPasteInstructions } from '../utils/clack';
 import { getFirstMatchedPath } from './glob';
 import { RN_SDK_PACKAGE } from './react-native-wizard';
+import { preserveTrailingNewline } from '../utils/ast-utils';
 
 // @ts-expect-error - magicast is ESM and TS complains about that. It works though
 import { generateCode, ProxifiedModule, parseModule } from 'magicast';
@@ -248,7 +249,8 @@ export async function wrapRootComponent() {
 
   traceStep('add-sentry-wrap', () => {
     try {
-      fs.writeFileSync(jsPath, generateCode(mod.$ast).code, 'utf-8');
+      const code = preserveTrailingNewline(js, generateCode(mod.$ast).code);
+      fs.writeFileSync(jsPath, code, 'utf-8');
       clack.log.success(
         `Added ${chalk.cyan('Sentry.wrap')} to ${chalk.cyan(jsRelativePath)}.`,
       );

src/react-router/codemods/react-router-config.ts

@@ -10,7 +10,7 @@ import { parseModule, generateCode } from 'magicast';
 import clack from '@clack/prompts';
 import chalk from 'chalk';
 
-import { findProperty } from '../../utils/ast-utils';
+import { findProperty, preserveTrailingNewline } from '../../utils/ast-utils';
 
 /**
  * Extracts the ObjectExpression from various export patterns.
@@ -205,7 +205,10 @@ export default {
     throw new Error('Failed to modify React Router config structure');
   }
 
-  const code = generateCode(mod.$ast).code;
+  const code = preserveTrailingNewline(
+    configContent,
+    generateCode(mod.$ast).code,
+  );
   await fs.promises.writeFile(configPath, code);
 
   return { ssrWasChanged };

src/react-router/codemods/vite.ts

@@ -10,7 +10,11 @@ import { parseModule, generateCode } from 'magicast';
 import clack from '@clack/prompts';
 import chalk from 'chalk';
 
-import { hasSentryContent, findProperty } from '../../utils/ast-utils';
+import {
+  hasSentryContent,
+  findProperty,
+  preserveTrailingNewline,
+} from '../../utils/ast-utils';
 
 /**
  * Extracts ObjectExpression from function body.
@@ -199,7 +203,10 @@ export async function instrumentViteConfig(
     throw new Error('Failed to modify Vite config structure');
   }
 
-  const code = generateCode(mod.$ast).code;
+  const code = preserveTrailingNewline(
+    configContent,
+    generateCode(mod.$ast).code,
+  );
   await fs.promises.writeFile(configPath, code);
 
   return { wasConverted };

src/sourcemaps/tools/vite.ts

@@ -28,7 +28,11 @@ import {
   SourceMapUploadToolConfigurationFunction,
   SourceMapUploadToolConfigurationOptions,
 } from './types';
-import { findFile, hasSentryContent } from '../../utils/ast-utils';
+import {
+  findFile,
+  hasSentryContent,
+  preserveTrailingNewline,
+} from '../../utils/ast-utils';
 
 import * as path from 'path';
 import * as fs from 'fs';
@@ -163,7 +167,10 @@ export async function addVitePluginToConfig(
       },
     });
 
-    const code = generateCode(mod.$ast).code;
+    const code = preserveTrailingNewline(
+      viteConfigContent,
+      generateCode(mod.$ast).code,
+    );
 
     await fs.promises.writeFile(viteConfigPath, code);
 

src/sveltekit/sdk-setup/vite.ts

@@ -14,7 +14,10 @@ import * as recast from 'recast';
 import x = recast.types;
 import t = x.namedTypes;
 
-import { hasSentryContent } from '../../utils/ast-utils';
+import {
+  hasSentryContent,
+  preserveTrailingNewline,
+} from '../../utils/ast-utils';
 import { debug } from '../../utils/debug';
 import { abortIfCancelled } from '../../utils/clack';
 import type { ProjectInfo } from './types';
@@ -64,7 +67,10 @@ Skipping adding Sentry functionality to.`,
 
     await modifyAndRecordFail(
       async () => {
-        const code = generateCode(viteModule.$ast).code;
+        const code = preserveTrailingNewline(
+          viteConfigContent,
+          generateCode(viteModule.$ast).code,
+        );
         await fs.promises.writeFile(viteConfigPath, code);
       },
       'write-file',

src/utils/ast-utils.ts

@@ -329,3 +329,26 @@ export function findProperty(
       p.key.name === name,
   ) as t.ObjectProperty | undefined; // Safe: predicate guarantees type
 }
+
+/**
+ * Preserves trailing newline from original content if present.
+ * Code generators like magicast/recast don't preserve trailing newlines,
+ * so this ensures we don't unnecessarily modify user files.
+ *
+ * @param originalContent - The original file content
+ * @param generatedCode - The code generated by AST transformation
+ * @returns The generated code with trailing newline preserved if original had one
+ */
+export function preserveTrailingNewline(
+  originalContent: string,
+  generatedCode: string,
+): string {
+  const hadTrailingNewline = originalContent.endsWith('\n');
+  const hasTrailingNewline = generatedCode.endsWith('\n');
+
+  if (hadTrailingNewline && !hasTrailingNewline) {
+    return generatedCode + '\n';
+  }
+
+  return generatedCode;
+}

test/sourcemaps/tools/vite.test.ts

@@ -52,7 +52,8 @@ export default defineConfig({
   build: {
     sourcemap: true
   }
-})`,
+})
+`,
     ],
     [
       'no build.sourcemap options',
@@ -62,10 +63,10 @@ export default defineConfig({
     vue(),
   ],
   build: {
-    test: 1,  
+    test: 1,
   }
 })
-  `,
+`,
       `import { sentryVitePlugin } from "@sentry/vite-plugin";
 export default defineConfig({
   plugins: [vue(), sentryVitePlugin({
@@ -76,7 +77,8 @@ export default defineConfig({
     test: 1,
     sourcemap: true
   }
-})`,
+})
+`,
     ],
     [
       'keep sourcemap: "hidden"',
@@ -89,7 +91,7 @@ export default {
     sourcemap: "hidden",
   }
 }
-    `,
+`,
       `import { sentryVitePlugin } from "@sentry/vite-plugin";
 export default {
   plugins: [vue(), sentryVitePlugin({
@@ -99,7 +101,8 @@ export default {
   build: {
     sourcemap: "hidden",
   }
-}`,
+}
+`,
     ],
     [
       'rewrite sourcemap: false to true',
@@ -114,7 +117,7 @@ const cfg = {
 }
 
 export default cfg;
-      `,
+`,
       `import { sentryVitePlugin } from "@sentry/vite-plugin";
 const cfg = {
   plugins: [vue(), sentryVitePlugin({
@@ -127,7 +130,8 @@ const cfg = {
   }
 }
 
-export default cfg;`,
+export default cfg;
+`,
     ],
   ])(
     'adds the plugin and enables source maps generation (%s)',

test/utils/ast-utils.test.ts

@@ -3,6 +3,7 @@ import {
   getOrSetObjectProperty,
   hasSentryContent,
   parseJsonC,
+  preserveTrailingNewline,
   printJsonC,
   setOrUpdateObjectProperty,
 } from '../../src/utils/ast-utils';
@@ -221,6 +222,38 @@ describe('setOrUpdateObjectProperty', () => {
   });
 });
 
+describe('preserveTrailingNewline', () => {
+  it('adds trailing newline if original had one but generated does not', () => {
+    const original = 'const foo = 1;\n';
+    const generated = 'const foo = 1;';
+    expect(preserveTrailingNewline(original, generated)).toBe(
+      'const foo = 1;\n',
+    );
+  });
+
+  it('does not add trailing newline if original did not have one', () => {
+    const original = 'const foo = 1;';
+    const generated = 'const foo = 1;';
+    expect(preserveTrailingNewline(original, generated)).toBe('const foo = 1;');
+  });
+
+  it('does not double trailing newline if both already have one', () => {
+    const original = 'const foo = 1;\n';
+    const generated = 'const foo = 1;\n';
+    expect(preserveTrailingNewline(original, generated)).toBe(
+      'const foo = 1;\n',
+    );
+  });
+
+  it('does not remove trailing newline if original did not have one but generated does', () => {
+    const original = 'const foo = 1;';
+    const generated = 'const foo = 1;\n';
+    expect(preserveTrailingNewline(original, generated)).toBe(
+      'const foo = 1;\n',
+    );
+  });
+});
+
 describe('parse and print JSON-C', () => {
   it.each([
     ['simple JSON', "{'foo': 'bar'}"],