Exclude utility does not work in mapped types

I need exclude some literals for a mapped type:

type ReformValidator = "required" | "minLength" | "maxLength";

interface FieldsValidator {
    [key: string]: {
        [V in ReformValidator]?: any
    } & {
        [K in Exclude<string, ReformValidator>]?: {
            validator: () => boolean;
            message: string;
        }
    }
}

const validators: FieldsValidator = {
    firstName: {
        required: true
    }
}

Expecting no error, but has:

Type '{ required: true; }' is not assignable to type '{ required?: any; ... } & { [x: string]: { ... }; }'.

• Type '{ required: true; }' is not assignable to type '{ ... }; }'.
  • Property 'required' is incompatible with index signature.
    • Type 'boolean' is not assignable to type '{ validator: () => boolean; message: string; }'

First, if you don't want to be able to assign { validator: () => boolean; message: string; } to keys within ReformValidator, you have to restrict its type. It cannot be any, because { validator: () => boolean; message: string; } satisfies any. I'd suggest a more concrete type like boolean | number.

Second, replace & with | and there you go.

interface FieldsValidator {
    [key: string]: {
        [V in ReformValidator]?: boolean;
    } | {
        [K: string]: {
            validator: () => boolean;
            message: string;
        }
    }
}

You can get rid of ? in [K: string]?, because if the key of Record is string, then the values are not concrete, so they are not needed.

You can simplify the declaration even more by using built-in types:

type FieldsValidator = Record<
  string, 
  Partial<Record<ReformValidator, boolean | number>>
  | Record<string, {
    validator: () => boolean; 
    message: string
  }>
>;

Which means:

1. Record<K, V> is equivalent to { [key: K]: V }
2. Partial<K> makes all keys in K optional (with ?), so Partial<Record<K, V>> is equivalent to { [key: K]?: V }