TypeScript - Type Inference with keyof keyword

Question

I have an abstract class

export abstract class ABaseModel {
  public static isKeyOf<T>(propName: (keyof T)): string {
    return propName;
  }
}

And another class that extends it

export class Model extends ABaseModel {

  public id: number;
  public uid: string;
  public createdByUid: string;
}

To check if a string is a valid property of that class, I have to wirte

Model.isKeyOf<Model>('id');

But I want to write

Model.isKeyOf('id');

Isn't it possible with Type Inference?

Nitzan Tomer · Accepted Answer · 2017-03-24 16:04:07Z

3

This seems to work:

abstract class ABaseModel {
    public static isKeyOf<T>(this: { new(): T }, propName: (keyof T)): string {
        return propName;
    }
}

Model.isKeyOf("id"); // ok
Model.isKeyOf("name"); // error: Argument of type '"name"' is not assignable to parameter of type '"id" | "uid" | "createdByUid"'

(code in playground)

answered Mar 24, 2017 at 16:04

Nitzan Tomer

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3 Comments

ppulwey Over a year ago

Getting error: The 'this' context of type 'typeof Model' is not assignable to method's 'this' of type 'new () => Model'.

Nitzan Tomer Over a year ago

What's the signature of your Model class? It should be the same, for example if it's class Model { constructor(a: string, b: number) { ... } } then you should use: (this: { new(a: string, b: number): T }

ppulwey Over a year ago

The constructor is constructor(props: any[]) { super(); Object.assign(this, props); } So it works with public static isKeyOf<T>(this: { new ({ }): T }, propName: (keyof T)): string { return propName; } Great, thanks a lot!

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