How to enforce number of fields in a record?

[joeveiga]

I was wondering if there is a way to create a version of typescript Record<K, V> type where we can enforce only a single field is present. The field key is not known at compile time and can be anything (any random string in the example), but we can't have more than one.

type SingleFieldRecord<K, V> = any; // TODO

let test: SingleFieldRecord<string, number>;

test = { 'some.field': 1 }; // OK
test = { 'another.random.field': 1, 'extra.field': 2 }; // ERROR (too many fields)

[millsp]

I think it would be possible by inferring the keys, then using Union utilities to make that a Tuple, then return an error message if the size of the tuple is not Equal to 1.

[joeveiga]

Thanks @millsp! If you get chance to take a crack at it that'd be greatly appreciated. I'm just getting started playing with ts types and I think this is above my current level :). This is more or less my use case:

type Operator =
  | {
      // $stringIn: SingleFieldRecord<string, string[]>;
      $stringIn: {
        [key: string]: string[];
      };
    }
  | {
      // $numericGE: SingleFieldRecord<string, number>;
      $numericGE: {
        [key: string]: number;
      };
    };
  // ...

// e.g
const op: Operator = {
  $numericGE: {
    'customer.age': 18,
    // I'd like to prevent extra fields here since this api only
    // expects one single field per operator. e.g.

    'customer.height': 6 // this shouldn't be allowed 
  }
}