Typescript not correctly inferring type

Question

class A {
    readonly is_prop: boolean;
    constructor(is_prop: boolean) {
        this.is_prop = is_prop;
    }
}

class B {
    readonly my_prop: boolean;
    constructor(my_prop: boolean) {
        this.my_prop = my_prop;
    }
}

type A1 = A & {is_prop: true};
type A2 = A & {is_prop: false};

type C = A1 | A2 | B;

const a: A = new A(true);
const b: A = new A(false);
const c: B = new B(true);

const e: A1 | null = a.is_prop ? a : null;

In the above example, why is the assignment for e giving error? Why is TS not inferring that is_prop will be true

Typescript playground

Intersection types in typescript behave as function overloading as far as I know. So saying A & {is_prop: true}; equals to a type that has both is_prop : boolean and is_prop: true. They aren't actually unified, which is what I believe you want which would be type A1 = Omit<A, 'is_prop'> & { is_prop:true}; — sinanspd
– sinanspd, Commented Apr 13, 2021 at 4:33
It's something along the lines of that. I am not entirely sure of the correct utility that's why I didn't put it as an answer. The point is as far as I know you can't achieve this by simple intersection. You will need to use utility types — sinanspd
– sinanspd, Commented Apr 13, 2021 at 4:42

chrisbajorin · Accepted Answer · 2021-04-13 05:04:30Z

1

The error is because a is of type A:

const a: A = new A(true);

In the ternary, you check that is_prop is true, but the variable a is still of type A. The property has been narrowed to true, but a hasn't changed types. As an example, this code would be valid based on your narrowing:

const trueVal: true | null = a.is_prop ? a.is_prop : null

If you want to be able to narrow the type of a, you need to say that it can be one of several types by stating that it's a union:

const a: A1|A2 = new A(true);

playground with some more alternatives.

answered Apr 13, 2021 at 5:04

chrisbajorin

6,1733 gold badges23 silver badges35 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Shivam Singla Over a year ago

This is the true explanation of the actual and the desired result.

jeremy302 · Accepted Answer · 2021-04-13 04:52:16Z

0

You get that error because of this line of code

type A1 = A & {is_prop: true}; //for class a, is_prop is boolean

boolean is not a subset of true, so you can't assign class A to type A1:

you could try this:

type A1 = A & {is_prop: boolean};

or this:

const e: A1 | null = a.is_prop ? a as A1: null;

answered Apr 13, 2021 at 4:52

jeremy302

8388 silver badges11 bronze badges

1 Comment

Denis Frezzato Over a year ago

Please don't suggest unsafe castings when there are safe alternatives.

zixiCat · Accepted Answer · 2021-04-13 05:16:01Z

That's because that true doesn't equal boolean type

And I don't think you need to declare type of the variables a b c again

They can be directly implicitly declared through the following generics.

class A<T = boolean> {
  readonly is_prop: T extends boolean ? T : never;
  constructor(is_prop: T extends boolean ? T : never) {
    this.is_prop = is_prop;
  }
}

class B {
  readonly my_prop: boolean;
  constructor(my_prop: boolean) {
    this.my_prop = my_prop;
  }
}

type A1 = A & { is_prop: true };
type A2 = A & { is_prop: false };

type C = A1 | A2 | B;

const a = new A(true);
const b = new A(false);
const c = new B(true);

const e: A1 | null = a.is_prop ? a : null;

In this way, you don't have to declare type A1 A2 C and declare type A1 | null to e, and then you can get more simplified code like that

class A<T = boolean> {
  readonly is_prop: T extends boolean ? T : never;
  constructor(is_prop: T extends boolean ? T : never) {
    this.is_prop = is_prop;
  }
}

class B {
  readonly my_prop: boolean;
  constructor(my_prop: boolean) {
    this.my_prop = my_prop;
  }
}

const a = new A(true);
const b = new A(false);
const c = new B(true);

const e = a.is_prop ? a : null;

Collectives™ on Stack Overflow

Typescript not correctly inferring type

3 Answers 3

1 Comment

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related