Type Aliases

In TypeScript, type aliases create type definitions that can be reused throughout the code. This is unlike type unions and intersections, where the explicit type information is used repetitively.

Syntax

Type aliases require the type keyword and a name. They can be created in two ways.

• They can be set as an object that describes the members and their respective types.

type myType = {
  memberOne: string;
  memberTwo: number;
}

let favoriteNum: myType = {"my favorite number is ", 42}

• They can also refer to other known types, like a union type.

type myType = string | number;

let favoriteNum: myType = '42';

Later on, whenever the name is used, TypeScript will assume this refers to the aliased value. Type aliases are similar to interfaces in terms of syntax and behavior. However, a key difference is that an interface can have members added later on whereas type aliases can’t be changed after being defined.

Type Aliases with Type Guards

In this example, the StringsIsh type alias can be used in place of the type union with several members. When applied to the logAllStrings() function:

type StringsIsh = string | string[] | null | undefined;

function logAllStrings(values: StringsIsh) {
  if (values === null || typeof values === 'undefined') return;

  if (typeof values === 'string') {
    console.log(values);
  } else {
    values.forEach(logAllStrings);
  }
}

logAllStrings('hello'); // Ok
logAllStrings(['hello', 'world']); // Also Ok

The first type guard checks to see if values matches either null or undefined in the StringIsh type alias. Next, the if/else-statement checks for a base case of values being of type string. Otherwise, it recursively invokes logAllStrings again.

Type Aliases as Generics

Type aliases may be generic and may contain any type description, including:

• Primitives
• Literals
• Object types

The following Result<T> type may contain a value, T, or an object containing the value, { value: T }:

type Result<T> = T | { value: T };

let value: Result<string> = 'hello'; // Ok
let other: Result<string> = { value: 'world' }; // Also Ok