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.

Titian Cernicova-Dragomir · Accepted Answer · 2018-01-24 08:51:56Z

2

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]();

edited Jan 24, 2018 at 8:51

answered Jan 24, 2018 at 8:42

Titian Cernicova-Dragomir

253k37 gold badges464 silver badges394 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

user2331095 Over a year ago

Thanks. I have one more question: If mylibs has many other properties/utils, like: mylibs.utils.request, mylibs.constants.X, which solution is better ?

user2331095 Over a year ago

I finally figure out it: let VecType: 'Vec2' | 'Vec3' | 'Vec4' = 'Vec' + len as any;.

Titian Cernicova-Dragomir Over a year ago

Both solutions can be used .. but it's not ideal, I will update my answer shortly

Titian Cernicova-Dragomir Over a year ago

@user2331095, Yup that was what I was going to say as well :). You can use the index solution as well but the index will return any instead of new ()=>any

Collectives™ on Stack Overflow

Typescript dynamic access property

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related