TypeScript Code Style Guide

When to apply

• When writing new TypeScript code
• During code review of .ts files
• When refactoring existing code
• When generating code snippets, examples, or documentation for TypeScript
• When working with TypeScript-related blocks in Markdown, Vue.js, etc.
• When formatting or cleaning up code

Rules

Full code examples for all rules are in references/examples.md.

Rule: No semicolons at the end of statements

No semicolons at the end of statements.

Exception: TypeScript interfaces, types, and similar type definitions MUST use semicolons as property separators.

// Statements - no semicolons
const result = calculateValue()

// Interfaces - semicolons required
interface User {
  id: number;
}

type Config = {
  timeout: number;
}

See examples → No semicolons for more.

Rule: No method calls in conditional statements

Extract method calls to separate variables before using them in conditional statements (if, while, for, etc.).

See examples → No method calls.

Rule: Use explicit presence checks in typed code

When checking whether a typed value exists, use explicit comparisons such as value === undefined, value !== undefined, value === null, or value === ''.

Do not rely on truthy/falsy narrowing for presence checks in normal application code.

See examples → Explicit presence checks.

Rule: No function calls as arguments

Extract function calls to separate variables before using them as arguments to other functions.

Exception: In rare cases (e.g., validator schemas, DSL configurations, query builders, matcher APIs) when a nested call is necessary for readability, keep it on separate lines. See examples → Nested function calls.

See examples → No function calls as arguments.

Rule: Group one-line declarations together, separate multiline declarations

One-line const and let declarations should stay together without blank lines. Add a blank line when moving between one-line and multiline declarations, or between separate multiline declarations.

See examples → Group one-line declarations.

Rule: No unclear abbreviations in local names

Use clear local variable names. Avoid non-obvious abbreviations such as pv, prop, cat, or cfg when a full word is short and clearer.

Well-known technical identifiers such as id, db, URL, and HTML are fine.

See examples → Clear local names.

Rule: Inline JSDoc comments on one line

One-line JSDoc comments should be on the same line with /** and */.

See examples → Inline JSDoc comments.

Rule: Object key ordering

Shorthand properties come first, then regular properties. Both groups are ordered alphabetically.

const config = {
  isActive,
  timeout,
  apiKey: 'secret-key',
  baseUrl: 'https://api.example.com'
}

See examples → Object key ordering for full examples.

Rule: Expand multi-value object and array literals

Single-key objects and single-item arrays may stay on one line. Objects with more than one key and arrays with more than one item should use one entry per line.

See examples → Expand multi-value literals.

Rule: Separate multiline object values with blank lines

Multiline values in objects should be separated by blank lines.

See examples → Separate multiline object values.

Rule: No trailing commas

Never use trailing commas in arrays, objects, or other structures.

See examples → No trailing commas.

Rule: Compute derived data before return

When a handler or function needs derived data, compute it in a separate variable first and keep the final return simple and explicit.

Avoid rebuilding objects through ...spread just to replace one field when an explicit object is clearer.

See examples → Derived data before return.

Rule: Separate multiline blocks and trailing returns with blank lines

Multiline blocks (if/else, loops, try/catch, functions, etc.) should be separated from other code with blank lines, unless they are at the start or end of a parent block. Add the same blank line before return when earlier statements appear in the same block.

See examples → Separate multiline blocks.

Rule: Flat interface structure

Every interface (and named type alias) must have only one level of named properties. If a property's type is an object shape, extract it as a separate named interface.

// ✅ Correct - extracted interfaces
interface Brand {
  name: string;
  slug: string;
}

interface Item {
  brand: Brand;
  id: string;
  name: string;
}

// ❌ Wrong - nested object shapes inline
interface Item {
  id: string;
  brand: { name: string; slug: string; };
  name: string;
}

See examples → Flat interface structure for a deeply nested example.