Typescript dynamic access property

Question

Here is a demo:

class Vec2 {};
class Vec3 {};
class Vec4 {};

let mylibs = {
    Vec2: Vec2,
    Vec3: Vec3,
    Vec4: Vec4
};
let len = Math.round(Math.random() * 2) + 2;
let VecType = 'Vec' + len;
let randomVector = new mylibs[VecType]();

I want to create something by user input, the VecType is something I used to simulate user input. The code above works, and tsc will not throw any error. However in my vscode, it tell me something wrong.

I want to resolve this kind of error. Thanks.

Answer 1

As the error suggests one solution would be to add an index signature to the type so as to allow indexing by any string :

let mylibs : { [key: string]:  new () => any} = {
    Vec2: Vec2,
    Vec3: Vec3,
    Vec4: Vec4
};

Instead of any if you want to be more restrictive, you could use a base class of the Vec* types, or a union type of all property types ( Vec2|Vec3|Vec4 )

Another option, would be to not index by a generic string but rather by a string that is a key for mylibs . To construct such a string dynamically would involve a cast which would not be particularly safe:

let VecType: keyof typeof mylibs = ('Vec' + len) as any;
let randomVector = new mylibs[VecType]();