TS Index Signature

TS Dynamic Attribute

class Foo {
    hello = "hello";
    world = 1234;

    // This is an index signature:
    [propName: string]: string | number | undefined;
}

let instance = new Foo();

// Valid assigment
instance["whatever"] = 42;

// Has type 'string | number | undefined'.
let x = instance["something"];

Common errors

class Foo {
    static hello = "hello";
    static world = 1234;

    static [propName: string]: string | number | undefined;
}

// Valid.
Foo["whatever"] = 42;

// Has type 'string | number | undefined'
let x = Foo["something"];

Announcing TypeScript 4.3 | TypeScript

Today we're excited to announce the availability of TypeScript 4.3! If you're not yet familiar with TypeScript, it's a language that builds on JavaScript by adding syntax for static types. Tools like the TypeScript compiler can just erase TypeScript syntax, leaving you with clean readable JavaScript that works anywhere.

https://devblogs.microsoft.com/typescript/announcing-typescript-4-3/