8. Type Aliases

Type Aliases

We've been using object types and union types by writing them directly in type annotations. This is convenient, but it's common to want to use the same type more than once and refer to it by a single name.

A type alias is exactly that - a name for any type. The syntax for a type alias is:

type Point = {
  x: number;
  y: number;
};

 
// Exactly the same as the earlier example
function printCoord(pt: Point) {
  console.log("The coordinate's x value is " + pt.x);
  console.log("The coordinate's y value is " + pt.y);
}
 
printCoord({ x: 100, y: 100 });

You can actually use a type alias to give a name to any type at all, not just an object type. For example, a type alias can name a union type:

type ID = number | string;

Note that aliases are only aliases - you cannot use type aliases to create different/distinct “versions” of the same type. When you use the alias, it's exactly as if you had written the aliased type. In other words, this code might look illegal, but is OK according to TypeScript because both types are aliases for the same type:

type UserInputSanitizedString = string;
 
function sanitizeInput(str: string): UserInputSanitizedString {
  return sanitize(str);
}
 
// Create a sanitized input
let userInput = sanitizeInput(getInput());
 
// Can still be re-assigned with a string though
userInput = "new input";

— The TypeScript Handbook: Everyday Types

Exercises

• Ex. 8.1: Primitive Aliases