TypeScript interface that allows other properties

interface Options {
  darkMode?: boolean;
  [otherOptions: string]: unknown;
  }

If you want ActionPayload to accept any other property you can add an indexer:

interface ActionPayload {
    actionType: string;

    // Keys can be strings, numbers, or symbols.
    // If you know it to be strings only, you can also restrict it to that.
    // For the value you can use any or unknown, 
    // with unknown being the more defensive approach.
    [x: string | number | symbol]: unknown;
}

See https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#strict-object-literal-assignment-checking