TypeScript array of types

Question

I have the following interface:

export interface FooInterface {
    barAction(x: any) : void;
}

Then I'm implementing it:

class FooImplA implements FooInterface {
    barAction(x: any) : void {};
}

class FooImplB implements FooInterface {
    barAction(x: any) : void {};
}

class FooImplB implements FooInterface {
    barAction(x: any) : void {};
}

Now I want an array of the types FooImplA and FooImplB (not the instances).

Obviously, this works:

  let typeArray = [FooImplA, FooImplB];

But I want it strongly typed. The below is what I have in mind, which doesn't work:

  let typeArray: Array< (typeof FooInterface) > = [FooImplA, FooImplB];

I could be misinterpreting the question, but is let typeArray:FooInterface[] = ... not what you want? — Katana314
– Katana314, Commented Nov 14, 2016 at 14:18
let typeArray:FooInterface[] only works if you have = [new FooImplA(), new FooImplB()] — David
– David, Commented Nov 14, 2016 at 14:41
Oh, right - these are references to the types, not instances. I noticed when programming against Angular 2, we ended up using their interface "Type": export interface Type<T> extends Function { new (...args: any[]): T; }. That probably has certain limitations, though it might at least let you define the constructor arguments. — Katana314
– Katana314, Commented Nov 14, 2016 at 14:47

Nitzan Tomer · Accepted Answer · 2016-11-14 19:51:28Z

1

Using your code:

interface FooInterface {
    barAction(x: any) : void;
}

class FooImplA implements FooInterface {
    barAction(x: any) : void {};
}

class FooImplB implements FooInterface {
    barAction(x: any) : void {};
}

let typeArray = [FooImplA, FooImplB];

The type of the typeArray variable is: typeof FooImplA[], you can see that in playground by hovering your mouse over typeArray.
The reason is that the compiler infers the type of the array itself without needing to explicitly tell it that.

If you do want to have it in the code, then you can do something like:

type FooImplConstructor = { new (): FooInterface };

let typeArray: FooImplConstructor[] = [FooImplA, FooImplB];
// or
let typeArray = [FooImplA, FooImplB] as FooImplConstructor[];

answered Nov 14, 2016 at 19:51

Nitzan Tomer

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