Two different ways to provide runtime type support for the username field

io-ts brands.ts

import * as t from 'io-ts';
import { withMessage } from 'io-ts-types/lib/withMessage';
import { Either, either } from 'fp-ts/lib/Either';

interface UsernameBrand {
  readonly Username: unique symbol;
}

type ErrorOrString = Either<t.Errors, string>

export const Username2 = new t.Type<string, string, unknown>(
  'Username2',
  t.string.is,
  ( u, c ): ErrorOrString =>
    either.chain( t.string.validate( u, c ), ( s: string ): ErrorOrString => {
      const length = s.length;
      if( length <= 2 || length >= 16 ) {
        return t.failure( u, c, 'Invalid length' );
      } else if( !/^[a-z0-9]+$/i.test( s ) ) {
        return t.failure( u, c, 'Invalid characters' );
      } else {
        return t.success( s );
      }
    } ),
  String
);

export const Username = withMessage( t.brand(
  t.string,
  ( s ): s is t.Branded<string, UsernameBrand> => {
    const length = s.length;
    if( length <= 2 || length >= 16 ) {
      return false;
    } else {
      return /^[a-z0-9]+$/i.test( s );
    }
  },
  'Username',
), ( x ): string => 'Invalid username' );

export type Username = t.TypeOf<typeof Username>

export type Username2 = t.TypeOf<typeof Username2>

export const LoginRequest = t.type( {
  username: Username2,
  password: t.string,
} );

export type LoginRequest = t.TypeOf<typeof LoginRequest>