Index Type

In TypeScript, indexed access types allow you to retrieve the type of a specific property within another type. This feature enables dynamic referencing of a property's type within an object type.

Example:

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

type NameType = Person['name']; // NameType is of type string

In this example, the Person type has properties name, age, and alive. By using Person['name'], we extract the type of the name property, which is string, and assign it to NameType.

 

Applications of Indexed Access Types:

1. Using Union Types: You can extract the types of multiple properties simultaneously.

type NameOrAge = Person['name' | 'age']; // NameOrAge is of type string | number

2. Combining with the keyof Operator: Retrieve the types of all properties in an object.

type PersonValues = Person[keyof Person]; // PersonValues is of type string | number | boolean

3. Extracting Array Element Types: This technique is also useful for obtaining the type of elements within an array.

const people = [
  { name: 'Alice', age: 30 },
  { name: 'Bob', age: 25 },
];

type PersonType = typeof people[number]; // PersonType is { name: string; age: number }

By leveraging indexed access types, you can dynamically extract and reuse the types of specific properties within objects or arrays, enhancing the flexibility and reusability of your type definitions.