How can I add a property to a class dynamically in typescript?

Question

How can I add a property to a class in typescript?

export class UserInfo {
  public name:string;
  public age:number;
}

let u:UserInfo = new UserInfo();
u.name = 'Jim';
u.age = 10;
u.address = 'London'; // Failed to compile. Property 'address' does not exist on type 'UserInfo'.

How to achieve this?

What are you trying to achieve? The whole purpose of typescript is to have well-defined interfaces and classes so that you don't have surprises. Why can't UserInfo contain an (optional) address property? — k0pernikus
– k0pernikus, Commented Jul 3, 2017 at 9:49
Possible duplicate of How do I dynamically assign properties to an object in TypeScript? — k0pernikus
– k0pernikus, Commented Jul 3, 2017 at 9:54
@k0pernikus At running, I'd like to add other properties for it. — niaomingjian
– niaomingjian, Commented Jul 3, 2017 at 9:56

k0pernikus · Accepted Answer · 2017-07-03 10:05:42Z

4

You could use index signature:

export class UserInfo {
    [index: string]: any;
    public name: string;
    public age: number;
}

const u: UserInfo = new UserInfo();
u.name = "Jim";
u.age = 10;
u.address = "London";

console.log(u);

Will output:

$ node src/test.js
UserInfo { name: 'Jim', age: 10, address: 'London' }

Yet note that thereby you are loosing the strict typechecks and introduce potential bugs that a prone to happen in weakly typed languages.

answered Jul 3, 2017 at 10:05

k0pernikus

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1 Comment

Crusader Over a year ago

I have a generic type Facade<T extends Record<string, AbstractClass>> and want to access the members like in simple record: Facade.Member.method() - is it possible in typescript?

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