How can I detect a class instance with generics in typescript

Question

I'm trying to create a function that takes two parameters, the first is the class reference and the second is a function with only one parameter that is an instance of the class reference and the output is a specific type. I tried the code below, but the typescript shows me an unknown type for p, even though I hope p was an instance of Pair.

interface Point{
    x: number
    y: number
}
class Pair{
    key = 0
    value = 0
}
function set<Class extends { new (): Type }, Type>(
    inputClass: Class,
    constructor: (o: Type) => Point) {
    //implementation...
}
set(Pair, p => ({x: p.key, y: p.value}))
// Typescript says:
// (parameter) p: unknown
// Object is of type 'unknown'.(2571)

I wanted the typescript to know that p must be an instance of Pair

I was able to implement using java generics, but I still can't replicate in typescript. Follow the code:

public <K extends Pair, T extends Class<K>> void setRenderer(T classe, Function<K, Point> constructor){
    //implementation
}

Ivan V. · Accepted Answer · 2020-04-17 21:23:35Z

2

This should work:

function set<C>(
    inputClass: new ()=>C,
    constructor: (o: C) => Point) {
    //implementation...
}

Typescript Playground

answered Apr 17, 2020 at 21:23

Ivan V.

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Comments

TPReal · Accepted Answer · 2020-04-17 21:24:12Z

1

Generics don't work like this, type inference doesn't go this way.

Simple solution:

interface Point{
    x: number
    y: number
}
class Pair{
    key = 0
    value = 0
}
declare function set<Type>(constructor: (o: Type) => Point): void;

set<Pair>(p => ({ x: p.key, y: p.value }))

Or if you really want to pass in the class:

interface Point{
    x: number
    y: number
}
class Pair{
    key = 0
    value = 0
}
function set<Type>(
    inputClass: { new (): Type },
    constructor: (o: Type) => Point) {
    //implementation...
}
set(Pair, p => ({ x: p.key, y: p.value }))

answered Apr 17, 2020 at 21:24

TPReal

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1 Comment

Guilherme Rocha Over a year ago

Thanks, the second way works for me. In that case, I really need the class as a parameter.

TPReal · Accepted Answer · 2020-04-17 21:22:14Z

0

I'm not so sure why you need classes here.

You could achieve what you're looking for with interfaces, like this:

interface Point{
    x: number
    y: number
}

interface Pair{
    key:number
    value:number
}
const instance: Pair = {
    key : 0,
    value : 0
}

function set(value: Pair):Point {
    return  ({x: value.key, y: value.value})
}

Playground

Or if you really do want classes you can do the following:

Class Playground

edited Apr 17, 2020 at 21:22

TPReal

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answered Apr 17, 2020 at 21:01

Damian Green

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2 Comments

Guilherme Rocha Over a year ago

Thanks for answering. But I need something more dynamic, something that I can infer from the class reference. My objective is to create a Map associating Classes and their respective builders. Something like new Map <T, (a: T) => Point>.

Damian Green Over a year ago

so T will vary? You may wish to update your question a little. Perhaps you are tying to do something like this? stackoverflow.com/questions/50774790/…

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