Typescript access static attribute of generic type

Question

I have an abstract class Model with a static attribute and another generic class Controller<T extends Model>. I want to access the static attribute of Model in an instance of Controller. That should like this:

abstract class Model{
    static hasStatus: boolean = false;
}

class MyModel extends Model{
     static hasStatus = true;
}

class Controller<T extends Model>{
    constructor(){
        if(T.hasStatus)...
    }
}

But TS says 'T' only refers to a type, but is being used as a value here.

Is there an easy way to achieve this? Or should i subclass Controller for each Heritage of Model and implement a method to retrieve the value?

How does T get bound when creating a Controller?

Ted Hopp
– Ted Hopp

2016-12-11 19:23:14 +00:00
Commented Dec 11, 2016 at 19:23 — Ted Hopp
– Ted Hopp, Commented Dec 11, 2016 at 19:23

artem · Accepted Answer · 2016-12-11 19:50:40Z

18

There is no way to do that in typescript. Generic type parameters can only appear where types may appear in declarations, they are not accessible at runtime. The reason for that is simple - single javascript function is generated for each method of the generic class, and there is no way for that function to know which actual type was passed as generic type parameter.

If you need that information at runtime, you have to add a parameter to the constructor and pass a type yourself when calling it:

class Controller<T extends Model>{
    constructor(cls: typeof Model){
        if (cls.hasStatus) {
        }
    }
}

let c = new Controller<MyModel>(MyModel);

Here is how it looks when compiled to javascript to illustrate the point - there is nothing left of generic parameters there, and if you remove cls parameter there is no information about where hasStatus should come from.

var Controller = (function () {
    function Controller(cls) {
        if (cls.hasStatus) {
        }
    }
    return Controller;
}());
var c = new Controller(MyModel);

answered Dec 11, 2016 at 19:50

artem

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1 Comment

Brain2000 Over a year ago

I think typescript generics and class statics, especially when they are combined, needs to be overhauled. You can't do anything with them.

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