Google TypeScript Style Guide

Apply Google's TypeScript Style Guide conventions for consistent, maintainable TypeScript code.

Overview

This skill provides guidelines for writing TypeScript code following Google's internal style guide. It covers source file structure, language features, naming conventions, type annotations, and best practices specific to TypeScript development.

Source File Basics

File Encoding

Source files must be encoded in UTF-8.

Whitespace

• Use ASCII horizontal space (0x20) as the only whitespace character

• Use special escape sequences (' , " , \ , \n , \r , \t , etc.) instead of numeric escapes

• For non-printable characters, use hex/Unicode escapes with explanatory comments

// Good: clear unicode character const units = 'μs';

// Good: escape with comment const output = '\ufeff' + content; // byte order mark

File Structure

Files must contain sections in this order (separated by exactly one blank line):

• Copyright information (if present)

• @fileoverview JSDoc (if present)

• Imports

• Implementation

/**

• @fileoverview Description of file. Lorem ipsum dolor sit amet, consectetur
• adipiscing elit, sed do eiusmod tempor incididunt. */

import * as foo from './foo'; import {Bar} from './bar';

export class MyClass { }

Imports and Exports

Import Styles

Use appropriate import styles:

• Module imports: import * as foo from '...';

• for namespacing

• Named imports: import {SomeThing} from '...';

• for frequently used symbols

• Default imports: import SomeThing from '...';

• only when required by external code

• Side-effect imports: import '...';

• only for side-effects

// Good: module import for namespace import * as ng from '@angular/core';

// Good: named import for clear symbols import {describe, it, expect} from './testing';

// Only when needed import Button from 'Button';

Named Exports Only

Always use named exports. Never use default exports.

// Good: named export export class Foo { } export const BAR = 42;

// Bad: default export export default class Foo { }

Why? Default exports provide no canonical name, making refactoring difficult and allowing inconsistent import names.

Import Paths

• Use relative paths (./foo ) for files within the same project

• Use paths from root for cross-project imports

• Limit parent steps (../../../ ) to maintain clarity

import {Symbol1} from 'path/from/root'; import {Symbol2} from '../parent/file'; import {Symbol3} from './sibling';

Variables and Constants

Use const and let

Always use const or let . Never use var .

const foo = otherValue; // Use const by default let bar = someValue; // Use let when reassigning

var foo = someValue; // Never use var

Why? const and let are block-scoped like most languages. var is function-scoped and causes bugs.

One Variable Per Declaration

Declare only one variable per statement:

// Good let a = 1; let b = 2;

// Bad let a = 1, b = 2;

Arrays and Objects

Array Literals

Do not use the Array constructor. Use bracket notation:

// Good const a = [2]; const b = [2, 3]; const c = Array.from<number>({length: 5}).fill(0);

// Bad: confusing behavior const a = new Array(2); // [undefined, undefined] const b = new Array(2, 3); // [2, 3]

Object Literals

Do not use the Object constructor. Use object literals:

// Good const obj = {}; const obj2 = {a: 0, b: 1};

// Bad const obj = new Object();

Spread Syntax

Use spread for shallow copying and concatenating:

// Good: array spread const foo = [1, 2]; const foo2 = [...foo, 6, 7];

// Good: object spread const foo = {num: 1}; const foo2 = {...foo, num: 5};

Rules:

• Only spread iterables into arrays

• Only spread objects into objects

• Later values override earlier values

Destructuring

Use destructuring for unpacking values:

// Good: array destructuring const [a, b, c, ...rest] = generateResults(); let [, b, , d] = someArray;

// Good: object destructuring const {num, str = 'default'} = options;

For function parameters, keep destructuring simple (single level, shorthand properties only):

// Good function destructured({num, str = 'default'}: Options = {}) { }

// Bad: too deeply nested function nestedTooDeeply({x: {num, str}}: {x: Options}) { }

Classes

Class Declarations

Class declarations must not end with semicolons:

// Good class Foo { }

// Bad class Foo { };

Class Members

Use readonly

Mark properties never reassigned outside constructor with readonly :

class Foo { private readonly bar = 5; }

Parameter Properties

Use parameter properties instead of obvious initializers:

// Good class Foo { constructor(private readonly barService: BarService) {} }

// Bad: unnecessary boilerplate class Foo { private readonly barService: BarService;

constructor(barService: BarService) { this.barService = barService; } }

Field Initializers

Initialize fields where declared when possible:

// Good class Foo { private readonly userList: string[] = []; }

// Bad: unnecessary constructor class Foo { private readonly userList: string[];

constructor() { this.userList = []; } }

Visibility

• Limit visibility as much as possible

• Never use public modifier except for non-readonly parameter properties

• TypeScript symbols are public by default

// Good class Foo { bar = new Bar(); // public by default

constructor(public baz: Baz) {} // public modifier allowed }

// Bad class Foo { public bar = new Bar(); // unnecessary }

No Private Fields (#)

Do not use private fields (#ident ). Use TypeScript's private modifier instead:

// Good class Clazz { private ident = 1; }

// Bad class Clazz { #ident = 1; }

Why? Private identifiers cause size/performance regressions when downleveled and don't offer benefits over TypeScript's visibility.

Functions

Prefer Function Declarations

For named functions, prefer function declarations over arrow functions:

// Good function foo() { return 42; }

// Bad const foo = () => 42;

Arrow Functions

Use arrow functions for:

• Callbacks and nested functions

• When explicit type annotation required

• When you need to preserve this context

// Good: callback bar(() => { this.doSomething(); });

// Good: nested function with this class Foo { method() { setTimeout(() => { this.doWork(); }, 100); } }

Arrow Function Bodies

Use concise bodies when return value is used, block bodies otherwise:

// Good: return value used const numbers = [1, 2, 3].map(v => v * 2);

// Good: no return value myPromise.then(v => { console.log(v); });

// Good: explicit void myPromise.then(v => void console.log(v));

No Rebinding this

Do not use this in function expressions/declarations unless rebinding is needed (which is discouraged):

// Bad function clickHandler() { this.textContent = 'Hello'; } document.body.onclick = clickHandler;

// Good: use arrow function document.body.onclick = () => { document.body.textContent = 'hello'; };

Type Annotations

Inference

Rely on type inference where possible. Do not annotate when the type is obvious:

// Good: type is obvious const x = 5; const y = new Foo();

// Bad: redundant annotation const x: number = 5;

Return Types

Annotate return types on exported functions and public methods:

// Good export function foo(): string { return 'bar'; }

Type vs Interface

• Use interface for object shapes and class contracts

• Use type for unions, intersections, and mapped types

• Prefer interface when either would work

// Good: interface for object shape interface User { name: string; age: number; }

// Good: type for union type Status = 'success' | 'error' | 'pending';

Naming Conventions

Identifiers

Use these naming conventions:

• Classes/Interfaces/Types/Enums: UpperCamelCase

• Functions/Methods/Properties: lowerCamelCase

• Constants: CONSTANT_CASE

• Private properties: prefix with private

• Type parameters: single uppercase letter or UpperCamelCase

class MyClass {} interface UserData {} type Status = 'active' | 'inactive'; enum Color { RED, GREEN, BLUE }

function doSomething() {} const userName = 'Alice'; const MAX_COUNT = 100;

class Foo { private readonly internalState = 5; }

File Names

• Use lowercase with dashes: file-name.ts

• Match exported symbol when exporting single symbol

• Use .d.ts for declaration files

String Literals

Use Single Quotes

Use single quotes for ordinary strings:

// Good const message = 'Hello world';

// Bad const message = "Hello world";

Template Literals

Use template literals for:

• String interpolation

• Multi-line strings

• Complex concatenation

// Good const greeting = Hello ${name}; const multiline = Line 1 Line 2;

Type Coercion

Explicit Coercion

Use String() , Boolean() , Number() for type coercion:

// Good const bool = Boolean(value); const str = String(num); const num = Number(str);

No Implicit Enum Coercion

Never convert enums to booleans implicitly. Compare explicitly:

enum SupportLevel { NONE, BASIC, ADVANCED }

// Bad if (level) { }

// Good if (level !== SupportLevel.NONE) { }

Quick Reference

DO

• Use const and let , never var

• Use named exports only

• Use readonly for immutable properties

• Use parameter properties

• Initialize fields where declared

• Use single quotes for strings

• Use template literals for interpolation

• Annotate return types on public functions

• Prefer interface over type for objects

DON'T

• Use default exports

• Use var for variables

• Use Array() or Object() constructors

• Use private fields (# )

• Rebind this unnecessarily

• Use function expressions (use arrows instead)

• Mix quoted and unquoted object keys

• Coerce enums to booleans

Additional Resources

Reference Files

For detailed specifications:

• references/language-features.md

• Detailed coverage of TypeScript language features

• references/type-system.md

• Type annotations, generics, utility types

• references/naming-conventions.md

• Complete naming rules and examples

Example Files

Working examples in examples/ :

• examples/class-example.ts

• Well-structured class with best practices

• examples/function-example.ts

• Function declarations and arrow functions

• examples/types-example.ts

• Type definitions and interfaces