What are literal types and how are they used for precise typing?

Literal types narrow a broad primitive type to a specific value. "circle" is a subtype of string, 42 is a subtype of number.

// String literals
type Direction = 'north' | 'south' | 'east' | 'west';
function move(dir: Direction) { /* only 4 valid inputs */ }

move('north'); // ✅
move('up');    // ❌ Error

// Numeric literals
type DiceRoll = 1 | 2 | 3 | 4 | 5 | 6;

// Boolean literals — useful for discriminated unions
type Result<T> =
  | { success: true;  value: T }
  | { success: false; error: string };

function process(r: Result<number>) {
  if (r.success) console.log(r.value); // narrows via boolean literal
  else console.log(r.error);
}

const assertions — widen prevention:

// Without const — inferred as string
const dir = 'north'; // type: string (widened)

// With const — inferred as literal type
const dir2 = 'north' as const; // type: "north"

// Object with const — all properties become readonly literals
const config = { host: 'localhost', port: 3000 } as const;
// type: { readonly host: "localhost"; readonly port: 3000 }

// Array with const — tuple instead of array
const tuple = [1, 'a'] as const; // type: readonly [1, "a"]