Literal Types

So far, types like string and number describe a whole category of values. TypeScript can also describe a single, exact value as its own type — called a literal type. "left" isn't just a string; it can be the type "left", matching nothing else.

let direction: "left" = "left";

direction = "left";  // fine — matches the literal type exactly
direction = "right"; // Error: Type '"right"' is not assignable to type '"left"'.

A single literal type on its own isn't very useful — you'd rarely want a variable that can only ever be one specific value. Literal types show their real strength combined with union types, using | to list every value that should be allowed.

type Direction = "left" | "right";

let move: Direction = "left";  // fine
move = "right";                // fine
move = "up";
// Error: Type '"up"' is not assignable to type 'Direction'.

This pattern is everywhere in real APIs, because it documents and enforces the exact set of valid options right in the type signature. Compare that to typing the parameter as plain string, which would silently accept a typo like "lft" and only fail once the program is running.

function swipe(direction: "left" | "right") {
  console.log(`Swiped ${direction}`);
}

swipe("left");  // fine
swipe("up");
// Error: Argument of type '"up"' is not assignable to
// parameter of type '"left" | "right"'.

type Direction = "left" | "right"; combines two literal types using a ____ type.

Why prefer function swipe(direction: "left" | "right") over function swipe(direction: string)?

let status: "on" | "off" = "on"; can later be reassigned to "standby" without a compiler error.