リテラル型

let answer: "yes" | "no";
answer = "yes"; 
// Valid
// answer = "maybe"; 
// Error: Type '"maybe"' is not assignable to type '"yes" | "no"'.

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

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

type DiceRoll = 1 | 2 | 3 | 4 | 5 | 6;

let roll: DiceRoll;
roll = 4; // Valid
// roll = 7; 
// Error: Type '7' is not assignable to type '1 | 2 | 3 | 4 | 5 | 6'.

function setStatus(status: "active" | "inactive"): void {
  console.log(`The status is now ${status}`);
}

setStatus("active"); 
// Output: The status is now active
// setStatus("pending"); 
// Error: Type '"pending"' is not assignable to type '"active" | "inactive"'.

type TrafficLight = "red" | "yellow" | "green";

let light: TrafficLight = "red";
console.log(light);
 // Output: red

type Button = {
  label: string;
  size: "small" | "medium" | "large";
};

let submitButton: Button = {
  label: "Submit",
  size: "medium",
};

console.log(submitButton);

type Switch = "on" | "off";

let state: Switch;
state = "on";
state = "off";