TypeScript Fundamentals

`any`, `unknown` & `never`

any disables type checking, unknown means a value must be checked before use, and never represents impossible values or unreachable paths.

any opts out
unknown is safe uncertainty
never means impossible
Use unknown at boundaries
Use never to make missed cases fail loudly

The three escape/extreme types
Type	Meaning	Use
any	Compiler, stop checking this	Isolated migration or unavoidable interop
unknown	I do not know this yet	Untrusted input
never	This cannot happen	Exhaustiveness and unreachable code

Exhaustiveness with never

function assertNever(value: never): never {
  throw new Error(`Unexpected value: ${JSON.stringify(value)}`)
}

function render(state: LoadState<User>) {
  switch (state.kind) {
    case "loading": return "Loading"
    case "success": return state.data.name
    case "error": return state.error.message
    default: return assertNever(state)
  }
}

Adding a new variant now forces this switch to be updated.