类型化变量的 TypeScript 空对象

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 .

Answer 1

Caveats

Here are two worthy caveats from the comments.

Either you want user to be of type User | {} 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 = "foo@bar.com";

Here is a working example for you .

Answer 2

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
}

Answer 3

In my case Record<string, never> helps, it was recommended by eslint

Answer 4

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: ""}

Answer 5

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

Answer 6

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;
}

Answer 7

What i wanted is intellisense help for a chain of middlewares. Record<string, never> worked for me.

type CtxInitialT = Record<string, never>;
type Ctx1T = CtxInitialT & {
  name: string;
};
type Ctx2T = Ctx1T & {
  token: string;
};

cont ctx: CtxInitialT = {};
// ctx.name = ''; // intellisense error Type 'string' is not assignable to type 'never'
cont ctx1: Ctx1T = middleware1AugmentCtx(ctx);
// ctx1.name = 'ddd'; // ok
// ctx1.name1 = ''; // intellisense error Type 'string' is not assignable to type 'never'
cont ctx2: Ctx2T = middleware2AugmentCtx(ctx1);
// ctx2.token = 'ttt'; // ok
// ctx2.name1 = ''; // intellisense error Type 'string' is not assignable to type 'never'

Answer 8

user: USER

this.user = ({} as USER)

Answer 9

Record<Keys,Type>

Constructs an object type whose property keys are Keys and whose property values are Type. This utility can be used to map the properties of a type to another type.

 const user:Record<string,string>={}

Answer 10

空对象可以写为Record<string,never> ，因此实际上您的用户类型是空对象或用户

const user : User | Record<string, never> = {};

Answer 11

In case of

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

you can declare an empty object or only one field(for example, only Username); in that case, you will not have an error, which can be a problem.

I will prefer more {} as User version.