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

Question

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]
}

CertainPerformance · Accepted Answer · 2022-01-31 16:34:01Z

4

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];

answered Jan 31, 2022 at 16:34

CertainPerformance

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5 Comments

ABOS Over a year ago

date should also be typed, not just string.

LastryCoding Over a year ago

Thank you it works, how can I change your answer to accept Date instead of string if I need to?

CertainPerformance Over a year ago

Object keys cannot be dates. Object keys may only be strings, numbers, or Symbols. If you want to have a Date object as a "key", use a Map instead. Map<Date, User[]>

ABOS Over a year ago

could there be any checking on date string such as 12-02-1996?

CertainPerformance Over a year ago

@ABOS Only if the date string is manually constructed - see this other answer for how to type it - but that's likely to require a decent amount of additional code for no real type benefit IMO

Sunny · Accepted Answer · 2022-01-31 16:36:19Z

0

In that case, you can use the Record interface.

let formatedUsers: Record<number, User[]> = {0:[]};

You can also initialize the value like that.

answered Jan 31, 2022 at 16:36

Sunny

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Comments

Samathingamajig · Accepted Answer · 2022-01-31 16:37:25Z

0

This is more accurate since it requires keys to be number-number-number

const formattedUsers: Record<`${number}-${number}-${number}`, User[]> = {};

answered Jan 31, 2022 at 16:37

Samathingamajig

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