TypeScript Basics

Overview

These TypeScript patterns ensure type-safe, maintainable React code. Follow these guidelines for all frontend development.

Interface Definitions

Data Model Interfaces

interface Task {
  id: string;
  title: string;
  description: string | null;
  isCompleted: boolean;
  createdAt: string;
}

type TaskStatus = 'pending' | 'completed' | 'archived';

Request/Response Types

interface CreateTaskRequest {
  title: string;
  description?: string;
}

interface PagedResponse<T> {
  items: T[];
  page: number;
  totalCount: number;
}

Type Annotations

Function Parameters

function handleClick(id: string): void {
  console.log(id);
}

async function fetchTasks(): Promise<Task[]> {
  const response = await fetch('/api/tasks');
  return response.json();
}

State Types

const [tasks, setTasks] = useState<Task[]>([]);
const [error, setError] = useState<string | null>(null);

Event Handlers

function handleSubmit(e: React.FormEvent<HTMLFormElement>): void {
  e.preventDefault();
}

function handleChange(e: React.ChangeEvent<HTMLInputElement>): void {
  setValue(e.target.value);
}

Null and Undefined

Nullable Types

interface Task {
  description: string | null;  // Explicitly null
}

interface UpdateRequest {
  title?: string;  // Optional (undefined)
}

Null Checks

const title = task?.title ?? 'Untitled';

Utility Types

type TaskUpdate = Partial<Task>;           // All optional
type TaskSummary = Pick<Task, 'id' | 'title'>;  // Select props
type CreateTask = Omit<Task, 'id' | 'createdAt'>;  // Exclude props

Best Practices

Avoid any

// Bad
function processData(data: any) { ... }

// Good
function processData(data: Task[]) { ... }

Export Types

export interface Task { ... }
import type { Task } from '../types/task';