TIL: How to reference the item type of an array type?

I have one array type and needed to use the type of its item

Directly from AI (Sonnet 3.7)

In TypeScript, you can reference the type of an item in an array type in several ways:

Method 1: Use indexed access type

type Messages = {
  date: Date;
  content: string;
  userId: string;
  userName: string;
}[]

type MessageItem = Messages[number];

The number index accesses the type of elements in the array.

Method 2: Define the item type separately first

type MessageItem = {
  date: Date;
  content: string;
  userId: string;
  userName: string;
};

type Messages = MessageItem[];

This approach is often clearer, especially if you need to reuse the item type elsewhere in your code.

Method 3: Use ArrayType<T> utility type (custom)

You can also create a utility type if you need to do this frequently:

type ArrayItem<T> = T extends Array<infer U> ? U : never;

type Messages = {
  date: Date;
  content: string;
  userId: string;
  userName: string;
}[]

type MessageItem = ArrayItem<Messages>;

The first method (Messages[number]) is the most direct and commonly used approach to extract the item type from an array type.