Complete Guide to TypeScript Interfaces
TypeScript is a powerful tool designed to enhance code safety, especially in large-scale projects. **Interfaces** describe the shape of data, making your code more predictable and robust. In this guide, we'll cover all aspects of TypeScript interfaces, from basic definitions to advanced concepts like generics, composition, readonly properties, and discriminated unions, complete with practical examples.
Designing Interfaces
When designing interfaces, favor **descriptive and clear names**. Keep leaf nodes as primitive types (string, number, boolean) and maintain consistent naming and formatting for special data types like money, timestamps, and identifiers (ID). Use the **readonly** keyword to make your intent explicit for properties that should not change after creation. This improves code clarity and prevents accidental mutations.
From JSON to TypeScript Types
When consuming an API, you often start with a sample JSON response. To accelerate this process, use a tool like the JSON to TypeScript generator to automatically scaffold your initial interface drafts. Afterward, refine these drafts by improving names, narrowing overly broad types (e.g., from string | number to just string), and refactoring repeated structures into reusable interfaces. This approach allows you to safely consume data from APIs while maintaining a clean codebase.
Advanced Interface Concepts
The power of TypeScript extends beyond basic interfaces. **Generics** enable you to create flexible, reusable interfaces that can work with various types. **Composition (the & operator)** allows you to combine multiple interfaces into more complex types. **Discriminated Unions** are perfect for managing collections of objects with a common field (e.g., type) in a type-safe manner, which is especially useful for polymorphic data structures. It's also important to understand the difference between interfaces and **type** aliases and when to prefer one over the other.
Interoperability and Safety
When dealing with data from external sources (like an API), it's safest to initially assign it the **unknown** type. You should then validate and transform this data into safe, typed objects using a validation library like Zod. This prevents your application from crashing due to unexpected data structures. If you're migrating from .NET, the C# to TypeScript converter can provide a quick starting point, and the JSON Diff tool can help you track structural changes in your API responses.