Typescript empty object for a typed variable

Question

Say I have   type User             I want to create a new user but set it to be an empty object   const user  User          This fails saying property XX is missing const user  User      as any     This works but I don t want to use any   How do I do this  I don t want the variable to be null

User · Accepted Answer

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.

User · Answer

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

User · Answer

user  USER  this user       as USER

User · Answer

If you declare an empty object literal and then assign values later on  then you can consider those values optional  may or may not be there   so just type them as optional with a question mark   type User         Username   string      Email   string

User · Answer

you can do this as below in typescript   const  params      as any     params name   nazeh abel    since typescript does not behave like javascript so we have to make the type as any otherwise it won t allow you to assign property dynamically to an object

User · Answer

Note that using const user      as UserType just provides intellisense but at runtime user is empty object    and has no property inside  that means user Email will give undefined instead of     type UserType         Username  string      Email  string      So  use class with constructor for actually creating objects with default properties   type UserType       Username  string    Email  string      class User implements UserType     constructor         this Username           this Email              Username  string    Email  string     const myUser   new User    console log myUser      output   Username      Email      console log  val    myUser Email      output       You can also use interface instead of type  interface UserType     Username  string    Email  string          and rest of code remains same     Actually  you can even skip the constructor part and use it like this   class User implements UserType         Username          will be added to new obj       Email  string     will not be added    const myUser   new User    console log myUser      output   Username

[javascript] Typescript empty object for a typed variable

Caveats

Answer

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