How do you define an array of Partials in TypeScript?

Question

For instance,

type User = {
  first_name: string,
  last_name: string,
  notes: string
}

Which of the following would work?

const users:Partial<User>[] = [{first_name: "Name"}]; // A

const users:Partial<User[]> = [{first_name: "Name"}]; // B

const users:[Partial<User>] = [{first_name: "Name"}]; // C

Weirdly, all of these appear to be valid TypeScript at compile time.

Thanks in advance.

Answer 1

The proper way is

const users:Partial<User>[] = [{first_name: "Name"}];

Partial<User[]> means that it has some of Array properties.

[Partial<User>] means that it's an array of partial User with 1 element.

Weirdly, all of these appear to be valid TypeScript at compile time

The syntax is valid but types aren't. It's expected that Partial<User[]> causes an error because types are incompatible.

Answer 2

Option A gives you an array containing Partials:

const users:Partial<User>[] = [{first_name: "Name"}]; // A

If it helps you understand, think about how you would define an array of Partials if the type weren't generic. That would be Partial[].

If you remove the generic type from option B, you'd get a single Partial instead of an array.

Option C is a tuple rather than an array.