Typescript empty object for a typed variable

Really depends on what you're trying to do. Types are documentation in typescript, so you want to show intention about how this thing is supposed to be used when you're creating the type.

Option 1: If Users might have some but not all of the attributes during their lifetime

Make all attributes optional

type User = {
  attr0?: number
  attr1?: string
}

Option 2: If variables containing Users may begin null

type User = {
...
}
let u1: User = null;

Though, really, here if the point is to declare the User object before it can be known what will be assigned to it, you probably want to do let u1:User without any assignment.

Option 3: What you probably want

Really, the premise of typescript is to make sure that you are conforming to the mental model you outline in types in order to avoid making mistakes. If you want to add things to an object one-by-one, this is a habit that TypeScript is trying to get you not to do.

More likely, you want to make some local variables, then assign to the User-containing variable when it's ready to be a full-on User. That way you'll never be left with a partially-formed User. Those things are gross.

let attr1: number = ...
let attr2: string = ...
let user1: User = {
  attr1: attr1,
  attr2: attr2
}

Caveats

Here are two worthy caveats from the comments.

Either you want user to be of type User | {} or Partial<User>, or you need to redefine the User type to allow an empty object. Right now, the compiler is correctly telling you that user is not a User. – jcalz

I don't think this should be considered a proper answer because it creates an inconsistent instance of the type, undermining the whole purpose of TypeScript. In this example, the property Username is left undefined, while the type annotation is saying it can't be undefined. – Ian Liu Rodrigues

Answer

One of the design goals of TypeScript is to "strike a balance between correctness and productivity." If it will be productive for you to do this, use Type Assertions to create empty objects for typed variables.

type User = {
    Username: string;
    Email: string;
}

const user01 = {} as User;
const user02 = <User>{};

user01.Email = "[email protected]";

Here is a working example for you.