How to use value in type syntax

April 12, 2023

I want return type of my method is one of my class’s properties (dynamically), how can i do this?

Something like this:

interface IObject {
  [key: string]: any
}

class classX<T extends IObject> {
  public methodX(column: keyof T) {
    return (null as any) as T[`${column}`] // T['property']
  }
}
const varX = new classX<{ property: string }>()
varX.methodX('property') // expected type string

const varY = new classX<{ property: number }>()
varY.methodX('property') // expected type number

>Solution :

Just add a generic constraint to your method and use that to constrain both your parameter and return type:

interface IObject {
  [key: string]: any
}

class classX<T extends IObject> {
  public methodX<K extends keyof T>(column: K): T[K] {
    return (null as any) as T[K];
  }
}

const varX = new classX<{ property: string }>();
varX.methodX('property');

const varY = new classX<{ property: number }>();
varY.methodX('property');

^{Playground link}