TypeScript: Interface Vs. Type For Object Shapes

In the world of TypeScript, defining the shapes of your data is crucial for building robust and maintainable applications. You'll often encounter two primary ways to do this: using type aliases and interface declarations. While they can sometimes feel interchangeable, especially when defining simple object shapes, understanding their nuances and adhering to project-specific guidelines is key. This article dives into why you might choose one over the other, focusing on a common refactoring goal: refactoring to use interface instead of type for object shape definitions.

Why Refactor: The Case for interface

Many projects, including the one discussed in the docs/design/TypeScript Coding.md document, advocate for using interface when defining object structures. The primary motivation behind this recommendation is extensibility. Interfaces in TypeScript are inherently designed to be open and extensible. One of the most powerful features that interfaces offer over type aliases for object shapes is declaration merging. This means that if you declare an interface with the same name multiple times, TypeScript will merge their members. This capability is invaluable for several reasons:

• Augmenting Existing Interfaces: You can easily extend built-in JavaScript objects or third-party library types by declaring an interface with the same name in your own scope. For example, you might want to add a custom property to the global Window object for a browser-specific feature. With interface, this is straightforward and clean.
• Modular Design: In larger codebases, different modules might need to contribute to the shape of a particular object. Declaration merging allows each module to define its part of the interface without needing to coordinate a single, monolithic definition. This promotes a more decentralized and modular approach to defining types.
• Framework Integration: Many JavaScript frameworks and libraries leverage declaration merging extensively. By using interface for your object shapes, you seamlessly align with these patterns, making integration smoother and reducing potential type-related conflicts.

While type aliases are incredibly versatile and essential for other scenarios, when your primary goal is to define a contract for an object's shape that might need to be extended or merged later, interface is generally the preferred choice. This refactoring effort aims to align the codebase with its own documented best practices, leading to improved consistency, better maintainability, and ensuring that the project adheres to its intended architectural patterns. By making this change, we enhance the codebase's ability to evolve gracefully and support future extensions in a predictable manner.

The Proposed Change: From type to interface

So, what does this refactoring actually involve? The core task is to audit the codebase, paying close attention to files like src/domain/types.ts where many object shapes are currently defined using type aliases. The goal is to convert these type aliases into interface declarations wherever they are used to define the structure of an object. This is a straightforward, yet impactful, change. Let's look at a concrete example to illustrate the transformation:

Consider a type alias like this:

type CommandInfo = {
  name?: string;
  command: string;
  source: string;
  description?: string;
};

This type alias defines a shape for information related to a command, specifying properties like name, command, source, and description. To refactor this according to the proposed behavior, we would rewrite it as an interface:

interface CommandInfo {
  name?: string;
  command: string;
  source: string;
  description?: string;
}

Notice how the syntax changes from type TypeName = { ... } to interface TypeName { ... }. The outcome in terms of type checking is largely the same for this simple case. However, by adopting interface, we unlock the potential for declaration merging, which is the key differentiator and the reason for this refactoring. It's important to remember that type aliases are not being eliminated entirely. They remain crucial and irreplaceable for defining other types of constructs:

• Union Types: type Status = 'pending' | 'processing' | 'completed';
• Intersection Types: type UserWithAddress = User & Address;
• Mapped Types: type Readonly<T> = { readonly [P in keyof T]: T[P] };
• Primitive Types: type UserID = string | number;

Therefore, the refactoring process is selective. It targets type aliases that are exclusively used for defining object shapes. Other uses of type aliases should be preserved as they serve purposes that interface cannot fulfill. This ensures that we leverage the strengths of both constructs appropriately, leading to a cleaner, more idiomatic, and more extensible TypeScript codebase. The meticulous application of this rule will enhance the long-term maintainability and architectural integrity of the project.

What's Not Changing: Scope and Limitations

It's equally important to clarify what this refactoring effort does not entail to manage expectations and ensure everyone understands the scope of the task. This is a focused, stylistic, and structural adjustment within the codebase, not a comprehensive overhaul of development processes or tooling.

No Deep Code Comprehension or Advanced Tooling

This refactoring does not involve any attempt to deeply understand or