Custom Types

Introduction

You've already explored many of TypeScript's built-in features, from primitive types to arrays, and that's a fantastic achievement! However, the true power of TypeScript lies in its ability to create custom types, tailored specifically to your application's needs. Custom types go beyond what TypeScript provides by default, offering more flexibility and control in how you define and enforce types.

You've actually encountered a custom type before: tuples. For instance, a tuple type like [string, string, number, boolean] could be used to model user information on a website—first name, last name, age, and subscription status (paid or free).

Think of predefined types as simple ingredients—sometimes all you need is a string, just like you might sometimes just want a slice of bread. But when you combine different ingredients, you can create something much more complex and satisfying, like a sandwich. Similarly, custom types allow you to combine predefined types to create structures that precisely match your requirements.

Once you define custom types, you can use them throughout your code just like any other type. For example, you can use them as type annotations when declaring variables:

let user: UserType;

Or as part of your function definitions:

function processUser(data: UserType): ProcessedUserType {
  // Function implementation
}

TypeScript will also infer custom types where appropriate, just like it does with simpler types:

let processed = processUser(user);
// 'processed' is inferred to be of type 'ProcessedUserType'.

By leveraging custom types, you can create more organized, predictable, and maintainable code that fits your project's unique needs.