How to cast a class to object with only the variables as keys in typescript

Question

Say I have a class looking like below

class Model {
  id: string;
  _createdAt: string;
  _updatedAt: string;

  constructor(params: // currently I am doing Partial<Model> but I would like params to
// only contain the variables at the top excluding any methods/getters/setters){
    this.id = params.id;
    this._createdAt = params.createdAt;
    this._updatedAt = params.updatedAt;
  }

  get createdAt{
    return moment(this._createdAt);
  }

  get updatedAt{
    return moment(this._updatedAt);
  }
}

I would like to be able to construct a typescript type/interface of an object type that reflects the variables in the Model. I am currently doing params: Partial<Model> but this assumes that params is going to contain all the properties of Model class. However, I want only the variables to be the keys of the interface. Is there a way of doing this without having to define all properties of interface myself?

So you want the generated type to expect `id`, `_createdAt` and `_updatedAt` properties in this case ? — Frank Bessou, Jun 17 '21 at 09:00

score 2 · Accepted Answer · answered Jun 17 '21 at 10:12

Perhaps the read-only nature of the getter attribute can be used to exclude him

class Model {
  id="";
  _createdAt="";
  _updatedAt="";

  constructor(params:any ){
    this.id = params.id;
    this._createdAt = params.createdAt;
    this._updatedAt = params.updatedAt;
  }

  get createdAt(){
    return moment(this._createdAt);
  }

  get updatedAt(){
    return moment(this._updatedAt);
  }
}


type IfEquals<X, Y, A=X, B=never> =
  (<T>() => T extends X ? 1 : 2) extends
  (<T>() => T extends Y ? 1 : 2) ? A : B;

type WritableKeys<T> = {
  [P in keyof T]-?: IfEquals<{ [Q in P]: T[P] }, { -readonly [Q in P]: T[P] }, P>
}[keyof T];






type a  = Pick<Model,WritableKeys<Model>>

can you explain ` (() => T extends X ? 1 : 2) extends (() => T extends Y ? 1 : 2) ? A : B;` part a little bit for me? thanks — forJ, Jun 18 '21 at 01:48
@forJ https://stackoverflow.com/questions/49579094/typescript-conditional-types-filter-out-readonly-properties-pick-only-requir — 崮生1, Jun 19 '21 at 13:08

score 1 · Answer 2 · answered Jun 17 '21 at 11:05

I,

I think that the solution of 崮生1 is really great and correspond to your need. I just add a little change in is code to avoid the methods in the class :

type IfEquals<X, Y> = (<T>() => T extends X ? 1 : 2) extends <T>() => T extends Y
    ? 1
    : 2
    ? 'false'
    : 'true'

type WritableKeys<T> = {
    [P in keyof T]-?: IfEquals<{ [Q in P]: T[P] }, { -readonly [Q in P]: T[P] }> extends 'true'
        ? never
        : T[P] extends Function
        ? never
        : P
}[keyof T]

How to cast a class to object with only the variables as keys in typescript

2 Answers2