Type Aliases and Mapped Types in TypeScript

TypeScript is a powerful and flexible superset of JavaScript that adds static typing capabilities to the language. It offers several features to enhance the development experience and increase code reliability. Two particularly useful features are type aliases and mapped types. In this article, we will explore these features and understand how they can be leveraged in TypeScript.

Type Aliases

Type aliases allow developers to create custom names for existing types, making the code more readable and maintainable. They enable the creation of complex types by combining existing ones or defining unions and intersections.

To create a type alias, we use the type keyword followed by the desired name and the associated type definition. For example:

type Age = number;
type User = {
  name: string;
  age: Age;

In the above example, we define two type aliases: Age and User. Age is just an alias for the number type, while User is an alias for an object with properties name of type string and age of type Age. We can now use these aliases throughout our code:

function getUserAge(user: User): Age {
  return user.age;

Type aliases improve code readability by abstracting complex types and providing meaningful names that describe their purpose.

Mapped Types

Mapped types are a powerful feature of TypeScript that allows the transformation of one type into another by iterating over its properties and applying certain operations or modifications.

To define a mapped type, we use the keyof keyword, which represents the keys of an object type. We then apply the desired modifications using conditional types. For example:

type ReadonlyUser<T> = {
  readonly [P in keyof T]: T[P];

type Person = {
  name: string;
  age: number;

const readonlyPerson: ReadonlyUser<Person> = {
  name: 'John',
  age: 30,

readonlyPerson.age = 31; // Error: Cannot assign to 'age' because it is a read-only property.

In the above example, ReadonlyUser is a mapped type that takes an existing type T and transforms it into a new type where all properties are read-only. By using the keyof keyword, we iterate over each property of T and apply the read-only modifier.

Mapped types are particularly useful when dealing with immutable or readonly data structures, as they allow us to enforce immutability at the type level.


TypeScript's type aliases and mapped types are powerful features that enhance code readability and expressiveness. Type aliases allow us to create custom names for existing types, making our code more understandable, while mapped types provide a way to transform types based on their properties.

By leveraging these features, developers can write more maintainable and reliable code, ensuring that types are consistent and enforced throughout their applications. Whether it's creating complex types or manipulating existing ones, type aliases and mapped types are indispensable tools in the TypeScript developer's arsenal.

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