classes with static check methods. wanted to find the
matching one and return to it in wrap method.

this architecture perfectly suits for my need of OOP-ifying.
here is the way that I used to do it in JavaScript:

class A {
    static check = arg => typeof arg == "number"
    constructor(arg) { this.value = arg}
    log = () => console.log("A/", this.value)
}

class B {
    static check = arg => typeof arg == "string"
    constructor(arg) { this.note = arg }
    log = () => console.log("B/", this.note)
}

function wrap(arg, classes) {
    for (const Class of classes) 
        if (Class.check(arg))
            return new Class(arg)
}

const classes = [A, B]

wrap("adaptive", classes).log()
wrap(999, classes).log()

lately, I thought maybe TypeScript could help me with that. what
I expected was that the wrap call would indicate the return
value (since it’s all about types with no logic)

unfortunately, it was not the case. the closest I could get was that:

type PayloadA = number
type PayloadB = string
type PayloadGeneric = PayloadA | PayloadB

class A {
    static check(arg:PayloadGeneric): arg is PayloadA {
        return typeof arg == "number"
    }
    value: number
    constructor(arg: PayloadA) { this.value = arg}
    log = () => console.log("A/", this.value)
}

class B {
    static check(arg:PayloadGeneric): arg is PayloadB {
        return typeof arg == "string"
    }
    note: string
    constructor(arg: PayloadB) { this.note = arg}
    log = () => console.log("B/", this.note)
}

function wrap(arg: PayloadGeneric) {
    if (A.check(arg)) {   // understands that arg is number
        return new A(arg) // and that arg is suitable for PayloadA
    } 
    if (B.check(arg)) {   // understands that arg is string
        return new B(arg) // and that arg is suitable for PayloadB
    }
}

const instanceA = wrap(999) // but here it doesn't understand the type
const instanceB = wrap("adaptive") // neither here

(I know PayloadA and PayloadB may look silly since they’re just
number and string but the whole point is about having custom interfaces
and predicates. number and string is just for the example and wrapping them
into a type demonstrates my purpouse better)

look wrap can only accept PayloadA or PayloadB. inside, we have 2 ifs.
one checks for PayloadA and understands and returns, the other checks
for PayloadB and again understands well and returns. but TypeScript
doesn’t stop there and continues reading the rest, like it may get executed.
even thinks it may return undefined since I’m not handling the default
return, but logically, I do – I only accept A or B and handled both. so,

I’m handling all the cases but TypeScript doesn’t get it. how do I fix it? I want something like return type narrowing.

and please don’t tell me “just put return type A | B” I don’t wanna hardcode
cuz the given list can change. I may put classes that none of their check method
matches, in this case I’d like to see that the return type is going to be undefined.
or I put one that matches, then I’d like to see what the return type is gonna be.

thanks in advance