How to associate and infer property types in typescript from given value

September 13, 2022

I have the following example code that resembles the actual classes (I do not put the full class because is quite big.

type accepted = 'a' | 'b';

type objects = A | B;

class A {
    aprop: string[] = [];
    add(): void {}
}

class B {
    myprop: string = '';
    anotherprop: number = 1;
    add(): void {}
}

class C<T extends objects> {
    prop1: accepted;
    prop2: T;
    constructor (prop1: accepted, prop2: T) {
        this.prop1 = prop1;
        this.prop2 = prop2;
    }
}

const right1 = new C('a', new A());
const right2 = new C('b', new B());
const wrong1 = new C('a', new B()); //should not be possible

What I want is to make sure that whenever I instantiate a class C with certain value of prop1 (a set of possible strings) the prop2 shall be its correspondent class.

In other words, I want to infer that prop2 is class A or B when the value of prop1 is 'a' or 'b'.

In addition, a playground

>Solution :

Since you only have two classes in this example, I think you should use the overloads example.

class C<T extends objects> {
    prop1: accepted;
    prop2: T;
    constructor(prop1: 'a', prop2: A);
    constructor(prop1: 'b', prop2: B);
    constructor (prop1: accepted, prop2: T) {
        this.prop1 = prop1;
        this.prop2 = prop2;
    }
}

Playground

If you have more classes, you can use a map like this:

type ClassMap = {
    a: A;
    b: B;
}

class C<T extends accepted> {
    prop1: T;
    prop2: ClassMap[T];
    constructor (prop1: T, prop2: ClassMap[T]) {
        this.prop1 = prop1;
        this.prop2 = prop2;
    }
}

Playground