TypeScript Aliases

Note: If you're new to TypeScript, check our Getting Started with TypeScript tutorial first.

In TypeScript, a type alias is a way to create a new name for a type.

type Age = number;
const age: Age = 25;  

console.log(age);

Syntax of Type Alias

You can create a type alias using the type keyword, followed by the alias name and the type it represents.

type AliasName = Type;

For example,

type userName = string; 

Here,

• userName is the alias name.
• string is the type that userName represents.

Alias for Object Types

You can use a type alias to define the structure of an object. For example,

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

const person: Person = {
  name: "Alice",
  age: 30,
};

Here, type Person = { name: string; age: number; }; creates a type alias Person for an object with the name and age properties.

Then, we define a variable person with the Person type.

This is especially useful when an object type is used multiple times, as it allows you to reuse the Person type throughout your code.

Alias for Function Types

You can also use type aliases to define functions.

Let's define a type alias named Adder for a function that takes two numbers (num1 and num2) as input and returns a number:

type Adder = (num1: number, num2: number) => number;

Then, use the alias to define the function:

const add: Adder = (x, y) => x + y;

Here, we create a function called add() and we tell TypeScript that add() must match the Adder type (i.e, it must take two numbers and return a number).

Finally, call the function:

console.log(add(5, 10)); // Output: 15

Here's the complete program to add two numbers using a type alias for the function:

// Define a type alias for a function that takes two numbers and returns a number
type Adder = (a: number, b: number) => number;

// Use the alias to type a function
const add: Adder = (x, y) => x + y;

console.log(add(5, 10)); // Output: 15

type ID = string | number;

let userId: ID;
userId = 101;      // valid
userId = "abc123"; // valid