Literal Types

Literal Types

A literal type pins a variable to a specific value, not just a general type:

let direction: "north" | "south" | "east" | "west";
direction = "north";  // OK
direction = "up";     // Error: '"up"' is not assignable to type '"north" | "south" | "east" | "west"'

Literal types are string, number, or boolean values used as types.

Why Literal Types?

They let you describe a limited set of valid values — like an enum, but lighter:

type Status = "pending" | "active" | "closed";
type Coin = "heads" | "tails";
type HttpMethod = "GET" | "POST" | "PUT" | "DELETE";

Using Literal Types in Functions

type Direction = "up" | "down" | "left" | "right";

function move(dir: Direction, steps: number): string {
    return "Move " + steps + " steps " + dir;
}

console.log(move("up", 3));    // Move 3 steps up
console.log(move("left", 1));  // Move 1 steps left

as const for Literal Types

When you declare a variable with const, TypeScript infers a literal type for primitives:

const x = "hello";  // type is "hello", not string

But objects and arrays are still mutable, so TypeScript widens their types:

const config = { mode: "dark", level: 5 };
// type is { mode: string; level: number } — not literal

Use as const to make the entire value deeply readonly with literal types:

const config = { mode: "dark", level: 5 } as const;
// type is { readonly mode: "dark"; readonly level: 5 }

This is especially useful for arrays that should be treated as tuples:

const pair = [1, 2] as const;       // readonly [1, 2]
const dirs = ["up", "down"] as const; // readonly ["up", "down"]

Your Task

1. Define type Suit = "clubs" | "diamonds" | "hearts" | "spades". Write function cardSuit(suit: Suit): string that returns "The suit is: [suit]".
2. Create a const COLORS = ["red", "green", "blue"] as const. Write function describeColors(): string that returns the length and first element: "3 colors, first is red".