Advanced TypeScript

`satisfies`, `as const` & Literal Types

Literal types preserve exact values. as const freezes inference to readonly literal shapes. satisfies checks a value against a target type without widening away its precise inferred type.

Literal types represent exact values
as const preserves literals deeply
satisfies checks without replacing inference
This is ideal for configuration objects
as Type is not the same

`satisfies` keeps exact keys

type Route = { path: string; auth: boolean }

const routes = {
  home: { path: "/", auth: false },
  admin: { path: "/admin", auth: true },
} satisfies Record<string, Route>

type RouteName = keyof typeof routes
// "home" | "admin"

The object is checked as a route map, but its exact keys are preserved.

`as const` for literal arrays

const statuses = ["idle", "loading", "success", "error"] as const

type Status = typeof statuses[number]
// "idle" | "loading" | "success" | "error"

`as const` turns a normal string array into a source of literal union types.

Literal-preserving tools
Tool	What it does	Use for
literal type	Exact value type	Variants and constants
as const	Readonly literal inference	Arrays/config values
satisfies	Check shape, keep inference	Config maps
as Type	Force compiler belief	Last resort / interop