How to declare an object of arrays of custom type in typescript

I am searching how to declare a variable where I can store users by birthdate while avoiding using any

let formatedUsers = {} as "TYPE_NEEDED"
  for (let i = 0; i < users.length; i++) {
    const user= users[i];
    if (!formatedUsers[user.birthdate]) formatedUsers[user.birthdate] = [];
    formatedUsers[user.birthdate].push(user);
  }

In the end I want my variable "formatedUsers" to be like this:

formatedUsers = {
12-02-1996: [user1, user3, user4],
02-04-1998: [user2],
05-08-1999: [user5, user6]
}

>Solution :

The object keys are strings, and the object values are arrays of users, so a relatively simple Record will do it. Assuming you have a reference to the type of user, you can do:

const formattedUsers: Record<string, User[]> = [];

If you don’t have a reference to the user type, you can extract it first.

type User = (typeof users)[number];