Agfdhyk

A- Constructor Method Case

When I want to infer the generic class type from a constructor method parameter:

interface ObjectAsMap { [key: string]: boolean }



interface MyClass<T extends ObjectAsMap> {

  data: T



  constructor(data: T): MyClass<T>

}



class MyClass<T extends ObjectAsMap> {

  constructor(data: T) {

    this.data = data

  }

}



const a = new MyClass('Wrong parameter type')

const b = new MyClass({

  first: true,

  second: false,

})



console.log(b.data.first)

console.log(b.data.wrongProperty)

As expected, I get 2 errors:

• 
  new MyClass('Wrong parameter type') triggers Argument of type '"Wrong parameter type"' is not assignable to parameter of type 'ObjectAsMap'.
  


• 
  b.data.wrongProperty triggers Property 'wrongProperty' does not exist on type '{ first: true; second: false; }'.
  

B- Non-Constructor Method Case

Now, if I want to trigger exactly the same expected behavior from a non-constructor method:

interface ObjectAsMap { [key: string]: boolean }



interface MyClass<T extends ObjectAsMap> {

  data: T



  declare(data: T): MyClass<T>

}



class MyClass<T extends ObjectAsMap> {

  public data: T



  public declare(data: T) {

    this.data = data



    return this

  }

}



const myClassInstance = new MyClass()



const a = myClassInstance.declare('Wrong parameter type')

const b = myClassInstance.declare({

  first: true,

  second: false,

})



console.log(b.data.first)

console.log(b.data.wrongProperty)

I only get the first error:

• 
  myClassInstance.declare('Wrong parameter type') triggers Argument of type '"Wrong parameter type"' is not assignable to parameter of type 'ObjectAsMap'..

The b.data.wrongProperty should also trigger an error since this property does not exist within b#data. When I hover the mouse above b.data, it tells me (property) MyClass<ObjectAsMap>.data: ObjectAsMap instead of (property) MyClass<{ first: true; second: false; }>.data: { first: true; second: false; }.

Question

Is there a way to infer the parameter type in Case B like I do it in Case A ?

You just need to add an extra type parameter to capture the actual type of data in the call

interface ObjectAsMap { [key: string]: boolean }



interface MyClass<T extends ObjectAsMap> {

    data: T



    declar<U extends T>(data: U): MyClass<U>

}



class MyClass<T extends ObjectAsMap> {

    public data: T



    public declare<U extends T>(data: U): MyClass<U> {

        this.data = data



        return this as any

    }

}



const myClassInstance = new MyClass()



const a = myClassInstance.declare('Wrong parameter type')

const b = myClassInstance.declare({

    first: true,

    second: false,

})



console.log(b.data.first)

console.log(b.data.wrongProperty)