Typescript Guidelines

TypeScript Guidelines

TypeScript Version

• TypeScript 5.x+
• Strict mode enabled

Type Usage Rules

Avoid any Type

Use specific types or unknown instead of any:

// Avoid - loses type safety
function processData(data: any) {
    return data.value;
}

// Correct - use proper types
function processData(data: { value: string }) {
    return data.value;
}

// Correct - use unknown for truly unknown types
function processData(data: unknown) {
    if (typeof data === 'object' && data !== null && 'value' in data) {
        return (data as { value: string }).value;
    }
}

Use satisfies Instead of as

Prefer satisfies for type assertions:

// Correct - satisfies ensures type safety
const config = {
    url: '/api/data',
    method: 'GET',
} satisfies RequestConfig;

// Avoid - as bypasses type checking
const config = {
    url: '/api/data',
    method: 'GET',
} as RequestConfig;

When as is acceptable:

• Narrowing after type guards
• DOM element type assertions
• Event target casting

// Acceptable use of as
const target = event.target as HTMLInputElement;
const element = document.querySelector('.button') as HTMLButtonElement | null;

Type Inference

Let TypeScript infer types when possible:

// Correct - inferred types
const count = 0;  // number
const name = 'test';  // string
const items = [1, 2, 3];  // number[]

// Unnecessary - redundant type annotations
const count: number = 0;
const name: string = 'test';

When to annotate explicitly:

• Function parameters (always)
• Function return types (when complex or public API)
• Variables when inference would be too broad

// Explicit types needed
function calculateValue(input: number, offset: number): number {
    return input + offset;
}

// Explicit type prevents inference to never[]
const items: string[] = [];

Functional Programming Patterns

Prefer Array Methods Over Loops

Use array methods instead of for loops:

// Correct - use array methods
const doubled = numbers.map(n => n * 2);
const evens = numbers.filter(n => n % 2 === 0);
const sum = numbers.reduce((acc, n) => acc + n, 0);

// Create lookup maps
const usersById = users.reduce((acc, user) => {
    acc[user.id] = user;
    return acc;
}, {} as Record<string, User>);

// Avoid - imperative loops
const doubled = [];
for (let i = 0; i < numbers.length; i++) {
    doubled.push(numbers[i] * 2);
}

When loops are acceptable:

• Performance-critical hot paths (measure first)
• Breaking early from iteration
• Complex state machines

Immutability

Avoid mutation, create new objects:

// Correct - immutable operations
const newArray = [...oldArray, newItem];
const newObject = { ...oldObject, updatedField: newValue };
const filtered = items.filter(item => item.active);

// Avoid - mutating existing data
oldArray.push(newItem);
oldObject.updatedField = newValue;

Imports & Exports

Import Organization

Order and spacing:

// 1. External dependencies
import { something } from 'external-package';

// 2. Internal absolute imports (if using)
import { utility } from '@project/utils';

// 3. Relative imports
import { helper } from './utils/helper';
import { Type } from './types';

// Space after imports block
const myFunction = () => {};

Named Exports Only

Use named exports consistently:

// Correct - named export
export function calculateValue(value: number) {
    return value * 2;
}

export interface Config {
    url: string;
}

// Avoid default exports
export default function calculateValue(value: number) {
    return value * 2;
}

Benefits:

• Named exports enable better IDE refactoring
• Enforces consistent naming across imports
• Prevents confusion from different import names

Variable Declarations

Use const and let Only

// Correct
const MAX_VALUE = 10;
let currentValue = 0;

// Avoid legacy var keyword
var MAX_VALUE = 10;

Prefer const Over let

// Correct - use const when value doesn't change
const result = calculate(id);
const displayName = result?.name ?? 'Unknown';

// Avoid - unnecessary let
let result = calculate(id);

Type Definitions

Interface vs Type

Prefer interface for object shapes:

// Preferred for objects
export interface User {
    id: string;
    name: string;
    email: string;
}

// Use type for unions, intersections, utilities
export type UserId = string;
export type Status = 'active' | 'inactive' | 'error';
export type PartialUser = Partial<User>;

Generic Types

// Correct - generic function
function selectById<T extends { id: string }>(items: T[], id: string): T | undefined {
    return items.find(item => item.id === id);
}

// Usage
const user = selectById(users, 'user-1');

Enums vs Union Types

Prefer union types over enums:

// Correct - union type
type Status = 'pending' | 'success' | 'error';

function getStatus(): Status {
    return 'success';
}

// Avoid - enum (unless required by external API)
enum Status {
    Pending = 'pending',
    Success = 'success',
    Error = 'error',
}

Rationale: Union types are more lightweight and work better with TypeScript's type system.

Nullable Values

Optional vs Undefined vs Null

Prefer undefined over null:

// Correct
function findUser(id: string): User | undefined {
    return users.find(u => u.id === id);
}

// Avoid
function findUser(id: string): User | null {
    return users.find(u => u.id === id) ?? null;
}

Nullish Coalescing

// Correct - nullish coalescing
const timeout = config.timeout ?? 5000;
const name = user?.name ?? 'Unknown';

// Avoid - logical OR (handles falsy values incorrectly)
const timeout = config.timeout || 5000;  // 0 would be replaced!

Type Guards

Custom Type Guards

// Correct - type guard function
function isUser(value: unknown): value is User {
    return (
        typeof value === 'object' &&
        value !== null &&
        'id' in value &&
        'name' in value &&
        typeof (value as User).id === 'string' &&
        typeof (value as User).name === 'string'
    );
}

// Usage
if (isUser(data)) {
    console.log(data.name);  // TypeScript knows data is User
}

Utility Types

Common Utility Types

// Partial - make all properties optional
type PartialUser = Partial<User>;

// Required - make all properties required
type RequiredUser = Required<PartialUser>;

// Pick - select specific properties
type UserName = Pick<User, 'id' | 'name'>;

// Omit - exclude specific properties
type UserWithoutMeta = Omit<User, 'created_at' | 'updated_at'>;

// Record - object with specific key/value types
type UserMap = Record<string, User>;

// ReturnType - extract function return type
type UserReturn = ReturnType<typeof getUser>;

Error Handling

Error Types

// Correct - unknown error type
try {
    await fetchData();
} catch (error) {
    // error is unknown by default
    if (error instanceof Error) {
        console.error(error.message);
    } else {
        console.error('Unknown error', error);
    }
}

Known Gotchas

any Type

Avoid any at all costs - it disables type checking completely. Use unknown for truly unknown types, then narrow with type guards.

Type Assertion with as

Only use as for DOM elements and after type guards. Prefer satisfies for configuration objects.

Enum Runtime Overhead

Enums generate runtime code. Prefer union types which are compile-time only.

Default Exports

Never use default exports. They prevent consistent naming and break IDE refactoring.

Error Type in Catch Blocks

Errors in catch blocks are always unknown. Use type guards to narrow.

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