The `satisfies` Operator (Why It’s a Big Deal)

type Route = { path: string; auth: boolean };
 
const routes: Record<string, Route> = {
  login: { path: '/login', auth: false },
  dashboard: { path: '/dashboard', auth: true },
};
 
// Problem 1: routes.login.path is widened to string (not '/login').
// Problem 2: routes.settings does not error at compile-time because the index signature allows any string key!

type Route = { path: string; auth: boolean };
 
const routes = {
  login: { path: '/login', auth: false },
  dashboard: { path: '/dashboard', auth: true },
} satisfies Record<string, Route>;
 
// 1. Exact keys are preserved: routes.settings is a compile-time error!
// 2. Exact literals are preserved: routes.login.path is literal '/login', not just string.
// 3. Invalid shapes are caught immediately:
// const bad = { test: { path: 123 } } satisfies Record<string, Route>; // Error!

type RGB = [red: number, green: number, blue: number];
type Color = string | RGB;
 
const palette = {
  primary: '#3b82f6',
  accent: [16, 185, 129],
} satisfies Record<string, Color>;
 
// TypeScript validates that both are valid Colors, but still knows:
palette.primary.toUpperCase(); // Valid! TS knows primary is a string.
palette.accent.map((c) => c * 2); // Valid! TS knows accent is an array.

const x = { mode: 'prod' } as const;