key

Helpers for key-based guards and literal narrowing. Use narrowKeyTo to build reusable guards that constrain a specific property to a literal value.

import { narrowKeyTo, or, struct, isString, isNumber, oneOfValues } from 'is-kit';

type User = { id: string; age: number; role: 'admin' | 'guest' | 'trial' };

const isUser = struct({
  id: isString,
  age: isNumber,
  role: oneOfValues('admin', 'guest', 'trial'),
});

// Build role-specific guards that also narrow the 'role' field to literals
const byRole = narrowKeyTo(isUser, 'role');
const isAdmin = byRole('admin'); // Readonly<User> & { role: 'admin' }
const isGuest = byRole('guest'); // Readonly<User> & { role: 'guest' }
const isTrial = byRole('trial'); // Readonly<User> & { role: 'trial' }

// Compose as usual
const isGuestOrTrial = or(isGuest, isTrial);

declare const input: unknown;
if (isGuestOrTrial(input)) {
  // input.role is narrowed to 'guest' | 'trial'
}