TypeScript Indexed Access Types

TypeScript's Indexed Access Types allow you to access the type of a specific property within an object or array using bracket notation (Type[Key]). This feature provides precise type information about properties or elements, enhancing type safety when accessing deeply nested structures or specific object keys.

Syntax:

type ObjectKeyType = keyof ObjectType;
type PropertyType = ObjectType[ObjectKeyType];

Where -

• ObjectType is the object that we want to access the properties of. 
• ObjectKeyType is the type of key that we want to use to access the property. 
• PropertyType is the type of property that we want to access.

Example 1: In this example, we define a Book interface with properties like title, author, and year. Using TypeScript's indexed access types, we retrieve the title property's value from a Book instance.

// Define Book type
type Book = {
    title: string;
    author: string;
    year: number;
};

// Get type of title property
type PropertyType = Book["title"];

// Create Book instance
const book: Book = {
    title: "The Hitchhiker's Guide to the Galaxy",
    author: "Douglas Adams",
    year: 1979,
};

// Access title property
const title: PropertyType = book["title"];

console.log(title);

Output:

The Hitchhiker's Guide to the Galaxy

Example 2: In this example, we define a Book interface and use TypeScript's Indexed Access Types to access and modify the title property. We then update the title value in a Book instance.

// Define Book type
type Book = {
    title: string;
    author: string;
    year: number;
};

// Get type of title property
type PropertyType = Book["title"];

// Create Book instance
const book: Book = {
    title: "The Hitchhiker's Guide to the Galaxy",
    author: "Douglas Adams",
    year: 1979,
};

// Change title property
book["title"] = "Harry Potter and the Philosopher's Stone";


console.log(book);

Output:

{
  title: "Harry Potter and the Philosopher's Stone",
  author: 'Douglas Adams',
  year: 1979
}