diff --git a/.prettierrc.json b/.prettierrc.json
index 993c3270..2399e7ec 100644
--- a/.prettierrc.json
+++ b/.prettierrc.json
@@ -1,6 +1,6 @@
 {
   "bracketSpacing": false,
   "singleQuote": true,
-  "trailingComma": "es5",
+  "trailingComma": "all",
   "arrowParens": "avoid"
 }
diff --git a/src/clean.ts b/src/clean.ts
index b71be1ea..8dd3eebd 100644
--- a/src/clean.ts
+++ b/src/clean.ts
@@ -33,7 +33,7 @@ export async function clean(options: Options): Promise<boolean> {
     if (outDir === '.') {
       options.logger.error(
         `${chalk.red('ERROR:')} ${chalk.gray('compilerOptions.outDir')} ` +
-          'cannot use the value ".".  That would delete all of our sources.'
+          'cannot use the value ".".  That would delete all of our sources.',
       );
       return false;
     }
@@ -45,7 +45,7 @@ export async function clean(options: Options): Promise<boolean> {
     options.logger.error(
       `${chalk.red('ERROR:')} The ${chalk.gray('clean')} command` +
         ` requires ${chalk.gray('compilerOptions.outDir')} to be defined in ` +
-        'tsconfig.json.'
+        'tsconfig.json.',
     );
     return false;
   }
diff --git a/src/cli.ts b/src/cli.ts
index 4f8f137b..94e39305 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -42,7 +42,7 @@ export interface Options {
 export type VerbFilesFunction = (
   options: Options,
   files: string[],
-  fix?: boolean
+  fix?: boolean,
 ) => Promise<boolean>;
 
 const logger: Logger = console;
@@ -105,7 +105,7 @@ export async function run(verb: string, files: string[]): Promise<boolean> {
     throw new Error(
       `gts requires node.js 10.x or up. You are currently running
       ${process.version}, which is not supported. Please upgrade to
-      a safe, secure version of nodejs!`
+      a safe, secure version of nodejs!`,
     );
   }
 
@@ -133,7 +133,7 @@ export async function run(verb: string, files: string[]): Promise<boolean> {
       '**/*.js',
       '**/*.tsx',
       '**/*.jsx',
-      '--no-error-on-unmatched-pattern'
+      '--no-error-on-unmatched-pattern',
     );
   }
 
diff --git a/src/init.ts b/src/init.ts
index 98e75f78..09ebccfd 100644
--- a/src/init.ts
+++ b/src/init.ts
@@ -54,7 +54,7 @@ async function query(
   message: string,
   question: string,
   defaultVal: boolean,
-  options: Options
+  options: Options,
 ): Promise<boolean> {
   if (options.yes) {
     return true;
@@ -77,7 +77,7 @@ async function query(
 
 export async function addScripts(
   packageJson: PackageJson,
-  options: Options
+  options: Options,
 ): Promise<boolean> {
   let edits = false;
   const pkgManager = getPkgManagerCommand(options.yarn);
@@ -120,7 +120,7 @@ export async function addScripts(
 
 export async function addDependencies(
   packageJson: PackageJson,
-  options: Options
+  options: Options,
 ): Promise<boolean> {
   let edits = false;
   const deps: DefaultPackage = {
@@ -164,7 +164,7 @@ function formatJson(object: {}) {
 
 async function writePackageJson(
   packageJson: PackageJson,
-  options: Options
+  options: Options,
 ): Promise<void> {
   options.logger.log('Writing package.json...');
   if (!options.dryRun) {
@@ -186,7 +186,7 @@ export const ESLINT_IGNORE = 'build/\n';
 async function generateConfigFile(
   options: Options,
   filename: string,
-  contents: string
+  contents: string,
 ) {
   let existing;
   try {
@@ -209,7 +209,7 @@ async function generateConfigFile(
       `${chalk.bold(filename)} already exists`,
       'Overwrite',
       false,
-      options
+      options,
     );
   }
 
@@ -226,7 +226,7 @@ async function generateESLintConfig(options: Options): Promise<void> {
   return generateConfigFile(
     options,
     './.eslintrc.json',
-    formatJson(ESLINT_CONFIG)
+    formatJson(ESLINT_CONFIG),
   );
 }
 
@@ -265,7 +265,7 @@ insert_final_newline = true
 }
 
 export async function installDefaultTemplate(
-  options: Options
+  options: Options,
 ): Promise<boolean> {
   const cwd = process.cwd();
   const sourceDirName = path.join(__dirname, '../template');
@@ -287,7 +287,7 @@ export async function installDefaultTemplate(
   if (tsFiles.length !== 0) {
     options.logger.log(
       'Target src directory already has ts files. ' +
-        'Template files not installed.'
+        'Template files not installed.',
     );
     return false;
   }
@@ -310,7 +310,7 @@ export async function init(options: Options): Promise<boolean> {
       `${chalk.bold('package.json')} does not exist.`,
       'Generate',
       true,
-      options
+      options,
     );
 
     if (!generate) {
@@ -344,7 +344,7 @@ export async function init(options: Options): Promise<boolean> {
     cp.spawnSync(
       getPkgManagerCommand(options.yarn),
       ['install', '--ignore-scripts'],
-      {stdio: 'inherit'}
+      {stdio: 'inherit'},
     );
   }
 
diff --git a/src/util.ts b/src/util.ts
index 28c19287..edfe3a17 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -63,7 +63,7 @@ async function getBase(
   filePath: string,
   customReadFilep: ReadFileP,
   readFiles: Set<string>,
-  currentDir: string
+  currentDir: string,
 ): Promise<ConfigFile> {
   customReadFilep = customReadFilep || readFilep;
 
@@ -92,7 +92,7 @@ async function getBase(
         contents.extends,
         customReadFilep,
         readFiles,
-        path.dirname(filePath)
+        path.dirname(filePath),
       );
       contents = combineTSConfig(nextFile, contents);
     }
@@ -117,7 +117,7 @@ function combineTSConfig(base: ConfigFile, inherited: ConfigFile): ConfigFile {
   Object.assign(
     result.compilerOptions!,
     base.compilerOptions!,
-    inherited.compilerOptions!
+    inherited.compilerOptions!,
   );
   delete result.extends;
   return result;
@@ -161,7 +161,7 @@ export function getPkgManagerCommand(isYarnUsed?: boolean): string {
  */
 export async function getTSConfig(
   rootDir: string,
-  customReadFilep?: ReadFileP
+  customReadFilep?: ReadFileP,
 ): Promise<ConfigFile> {
   customReadFilep = (customReadFilep || readFilep) as ReadFileP;
   const readArr = new Set<string>();
diff --git a/test/kitchen.ts b/test/kitchen.ts
index df1888e2..9d03320a 100644
--- a/test/kitchen.ts
+++ b/test/kitchen.ts
@@ -82,7 +82,7 @@ describe('🚰 kitchen sink', () => {
     // Test package.json expects a gts tarball from ../gts.tgz.
     fs.copySync(
       path.join(stagingPath, 'gts.tgz'),
-      path.join(tmpDir.name, 'gts.tgz')
+      path.join(tmpDir.name, 'gts.tgz'),
     );
     // It's important to use `-n` here because we don't want to overwrite
     // the version of gts installed, as it will trigger the npm install.
@@ -91,12 +91,12 @@ describe('🚰 kitchen sink', () => {
     // The `extends` field must use the local gts path.
     const tsconfigJson = fs.readFileSync(
       path.join(tmpDir.name, 'kitchen', 'tsconfig.json'),
-      'utf8'
+      'utf8',
     );
     const tsconfig = JSON.parse(tsconfigJson);
     assert.deepStrictEqual(
       tsconfig.extends,
-      './node_modules/gts/tsconfig-google.json'
+      './node_modules/gts/tsconfig-google.json',
     );
 
     // server.ts has a lint error. Should error.
@@ -113,27 +113,27 @@ describe('🚰 kitchen sink', () => {
     assert.ok(
       fs
         .readFileSync(path.join(kitchenPath, 'package.json'), 'utf8')
-        .endsWith('\n')
+        .endsWith('\n'),
     );
     assert.ok(
       fs
         .readFileSync(path.join(kitchenPath, 'tsconfig.json'), 'utf8')
-        .endsWith('\n')
+        .endsWith('\n'),
     );
     assert.ok(
       fs
         .readFileSync(path.join(kitchenPath, '.eslintrc.json'), 'utf8')
-        .endsWith('\n')
+        .endsWith('\n'),
     );
     assert.ok(
       fs
         .readFileSync(path.join(kitchenPath, '.eslintignore'), 'utf8')
-        .endsWith('\n')
+        .endsWith('\n'),
     );
     assert.ok(
       fs
         .readFileSync(path.join(kitchenPath, '.prettierrc.js'), 'utf8')
-        .endsWith('\n')
+        .endsWith('\n'),
     );
   });
 
@@ -141,7 +141,7 @@ describe('🚰 kitchen sink', () => {
     const res = await execa(
       'npm',
       ['run', 'lint'],
-      Object.assign({}, {reject: false}, execOpts)
+      Object.assign({}, {reject: false}, execOpts),
     );
     assert.strictEqual(res.exitCode, 1);
     assert.ok(res.stdout.includes('assigned a value but'));
diff --git a/test/test-clean.ts b/test/test-clean.ts
index 0f039cbe..4c603da7 100644
--- a/test/test-clean.ts
+++ b/test/test-clean.ts
@@ -38,7 +38,7 @@ describe('clean', () => {
     return assert.rejects(() =>
       withFixtures({}, async () => {
         await clean(OPTIONS);
-      })
+      }),
     );
   });
 
@@ -73,7 +73,7 @@ describe('clean', () => {
       async () => {
         const deleted = await clean(OPTIONS);
         assert.strictEqual(deleted, false);
-      }
+      },
     );
   });
 
@@ -88,8 +88,8 @@ describe('clean', () => {
         async () => {
           const deleted = await clean(OPTIONS);
           assert.strictEqual(deleted, false);
-        }
-      )
+        },
+      ),
     );
   });
 
@@ -110,7 +110,7 @@ describe('clean', () => {
         assert.throws(() => {
           fs.accessSync(outputPath);
         });
-      }
+      },
     );
   });
 });
diff --git a/test/test-init.ts b/test/test-init.ts
index 7bc80b70..92d0a50f 100644
--- a/test/test-init.ts
+++ b/test/test-init.ts
@@ -45,7 +45,7 @@ function hasExpectedScripts(packageJson: PackageJson): boolean {
   return (
     !!packageJson.scripts &&
     ['lint', 'clean', 'compile', 'fix', 'prepare', 'pretest', 'posttest'].every(
-      s => !!packageJson.scripts![s]
+      s => !!packageJson.scripts![s],
     )
   );
 }
@@ -161,14 +161,14 @@ describe('init', () => {
         assert.notStrictEqual(
           contents,
           originalContents,
-          'the file should have been modified'
+          'the file should have been modified',
         );
         assert.strictEqual(
           contents.some,
           originalContents.some,
-          'unrelated property should have preserved'
+          'unrelated property should have preserved',
         );
-      }
+      },
     );
   });
 
@@ -195,7 +195,7 @@ describe('init', () => {
         const contents = await readJson('./package.json');
         const cmd = process.platform === 'win32' ? 'yarn.cmd' : 'yarn';
         assert.strictEqual(contents.scripts.prepare, cmd + ' run compile');
-      }
+      },
     );
   });
 
diff --git a/test/test-util.ts b/test/test-util.ts
index 8b735aea..20423267 100644
--- a/test/test-util.ts
+++ b/test/test-util.ts
@@ -42,7 +42,7 @@ function createFakeReadFilep(myMap: Map<string, ConfigFile>) {
 }
 
 function makeFakeFsExistsSync(
-  expected: PathLike[]
+  expected: PathLike[],
 ): (path: PathLike) => boolean {
   return (path: PathLike) => expected.some(item => item === path);
 }
@@ -58,7 +58,7 @@ describe('util', () => {
 
     function fakeReadFilep(
       configPath: string,
-      encoding: string
+      encoding: string,
     ): Promise<string> {
       assert.strictEqual(configPath, PATH_TO_TSCONFIG);
       assert.strictEqual(encoding, 'utf8');
@@ -82,7 +82,7 @@ describe('util', () => {
     return assert.rejects(
       () => getTSConfig(FAKE_DIRECTORY, createFakeReadFilep(myMap)),
       Error,
-      'Circular Reference Detected'
+      'Circular Reference Detected',
     );
   });
 
@@ -108,7 +108,7 @@ describe('util', () => {
 
     const contents = await getTSConfig(
       FAKE_DIRECTORY,
-      createFakeReadFilep(myMap)
+      createFakeReadFilep(myMap),
     );
     assert.deepStrictEqual(contents, combinedConfig);
   });
@@ -125,7 +125,7 @@ describe('util', () => {
 
     const contents = await getTSConfig(
       FAKE_DIRECTORY,
-      createFakeReadFilep(myMap)
+      createFakeReadFilep(myMap),
     );
     assert.deepStrictEqual(contents, combinedConfig);
   });
@@ -145,12 +145,12 @@ describe('util', () => {
     myMap.set(path.resolve(FAKE_DIRECTORY, './foo/FAKE_CONFIG2'), FAKE_CONFIG2);
     myMap.set(
       path.resolve(FAKE_DIRECTORY, './foo/bar/FAKE_CONFIG3'),
-      FAKE_CONFIG3
+      FAKE_CONFIG3,
     );
 
     const contents = await getTSConfig(
       FAKE_DIRECTORY,
-      createFakeReadFilep(myMap)
+      createFakeReadFilep(myMap),
     );
     assert.deepStrictEqual(contents, combinedConfig);
   });
@@ -162,7 +162,7 @@ describe('util', () => {
     return assert.rejects(
       () => getTSConfig(FAKE_DIRECTORY, createFakeReadFilep(myMap)),
       Error,
-      `${FAKE_DIRECTORY}/tsconfig.json Not Found`
+      `${FAKE_DIRECTORY}/tsconfig.json Not Found`,
     );
   });