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Type Aliases

The type keyword creates a named alias for any type - not just object shapes like interface, but also unions, primitives, tuples, and function signatures. Type aliases make complex types reusable and give them a meaningful name, improving both readability and error messages.

Syntax

type Name = TypeDefinition;

Examples

Aliasing Object Shapes and Unions

The two most common uses of type aliases.

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

type Status = "pending" | "active" | "completed"; // union of literal types

function updateStatus(status: Status) {
  console.log(`Status is now: ${status}`);
}

updateStatus("active");
// updateStatus("done"); // Error - "done" is not one of the allowed literals

Aliasing Function Signatures and Tuples

Type aliases work for more than just objects.

type MathFn = (a: number, b: number) => number;

const add: MathFn = (a, b) => a + b;
const multiply: MathFn = (a, b) => a * b;

type Coordinate = [number, number]; // a tuple alias

const origin: Coordinate = [0, 0];

Best practices

  • Use type aliases for unions, tuples, and function signatures - things interface cannot directly express
  • Give type aliases clear, descriptive names (Status, not T1) so error messages and autocomplete stay meaningful
  • For plain object shapes, either type or interface works - pick one convention and stay consistent within a project
  • Reuse a type alias across multiple functions/variables rather than repeating the same inline type definition

Runtime & versions

Compiled to JavaScript. Feature availability depends on the language and runtime version used by your project; consult the official reference below.

TypeScript Handbook

Specifications & further reading