So in the interface validationRegistry, the bracket means the index property but when does the [propName]: ['required'] mean in the function?

interface ValidatorConfig {
  [property: string]: {
    [validatableProp: string]: string[]; // ['required', 'positive']
  };
}

const registeredValidators: ValidatorConfig = {};

function Required(target: any, propName: string) {
  registeredValidators[target.constructor.name] = {
    ...registeredValidators[target.constructor.name],
    [propName]: ['required']
  };
}

function PositiveNumber(target: any, propName: string) {
  registeredValidators[target.constructor.name] = {
    ...registeredValidators[target.constructor.name],
    [propName]: ['positive']
  };
}