TypeScript Literal Types

// A variable with a string literal type
let direction: "north" | "south" | "east" | "west";

// Valid assignments
direction = "north";
direction = "south";

// Invalid assignments would cause errors
// direction = "northeast"; // Error: Type '"northeast"' is not assignable to type '"north" | "south" | "east" | "west"'
// direction = "up";        // Error: Type '"up"' is not assignable to type '"north" | "south" | "east" | "west"'

// Using string literal types in functions
function move(direction: "north" | "south" | "east" | "west") {
  console.log(`Moving ${direction}`);
}

move("east");  // Valid
// move("up");  // Error: Argument of type '"up"' is not assignable to parameter of type...

// A variable with a numeric literal type
let diceRoll: 1 | 2 | 3 | 4 | 5 | 6;

// Valid assignments
diceRoll = 1;
diceRoll = 6;

// Invalid assignments would cause errors
// diceRoll = 0;      // Error: Type '0' is not assignable to type '1 | 2 | 3 | 4 | 5 | 6'
// diceRoll = 7;      // Error: Type '7' is not assignable to type '1 | 2 | 3 | 4 | 5 | 6'
// diceRoll = 2.5;    // Error: Type '2.5' is not assignable to type '1 | 2 | 3 | 4 | 5 | 6'

// Using numeric literal types in functions
function rollDice(): 1 | 2 | 3 | 4 | 5 | 6 {
  return Math.floor(Math.random() * 6) + 1 as 1 | 2 | 3 | 4 | 5 | 6;
}

const result = rollDice();
console.log(`You rolled a ${result}`);

// A type that can only be the literal value 'true'
type YesOnly = true;

// A function that must return true
function alwaysSucceed(): true {
  // Always returns the literal value 'true'
  return true;
}

// Boolean literal combined with other types
type SuccessFlag = true | "success" | 1;
type FailureFlag = false | "failure" | 0;

function processResult(result: SuccessFlag | FailureFlag) {
  if (result === true || result === "success" || result === 1) {
    console.log("Operation succeeded");
  } else {
    console.log("Operation failed");
  }
}

processResult(true);      // "Operation succeeded"
processResult("success"); // "Operation succeeded"
processResult(1);         // "Operation succeeded"
processResult(false);     // "Operation failed"

// Object with literal property values
type HTTPSuccess = {
  status: 200 | 201 | 204;
  statusText: "OK" | "Created" | "No Content";
  data: any;
};

type HTTPError = {
  status: 400 | 401 | 403 | 404 | 500;
  statusText: "Bad Request" | "Unauthorized" | "Forbidden" | "Not Found" | "Internal Server Error";
  error: string;
};

type HTTPResponse = HTTPSuccess | HTTPError;

// A type guard is needed here: a numeric range check alone does not narrow the union
function isSuccess(response: HTTPResponse): response is HTTPSuccess {
  return response.status >= 200 && response.status < 300;
}

function handleResponse(response: HTTPResponse) {
  if (isSuccess(response)) {
    console.log(`Success: ${response.statusText}`);
    console.log(response.data);
  } else {
    console.log(`Error ${response.status}: ${response.statusText}`);
    console.log(`Message: ${response.error}`);
  }
}

// Example usage
const successResponse: HTTPSuccess = {
  status: 200,
  statusText: "OK",
  data: { username: "john_doe", email: "[email protected]" }
};

const errorResponse: HTTPError = {
  status: 404,
  statusText: "Not Found",
  error: "User not found in database"
};

handleResponse(successResponse);
handleResponse(errorResponse);

// Basic template literals
type Direction = "north" | "south" | "east" | "west";
type Distance = "1km" | "5km" | "10km";

// Using template literals to combine them
type DirectionAndDistance = `${Direction}-${Distance}`;
// "north-1km" | "north-5km" | "north-10km" | "south-1km" | ...

let route: DirectionAndDistance;
route = "north-5km";   // Valid
route = "west-10km";   // Valid
// route = "north-2km";   // Error
// route = "5km-north";   // Error

// Advanced string manipulation
type EventType = "click" | "hover" | "scroll";
type EventTarget = "button" | "link" | "div";
type EventName = `on${Capitalize<EventType>}${Capitalize<EventTarget>}`;
// "onClickButton" | "onClickLink" | "onClickDiv" | ...

// Dynamic property access
type User = {
  id: number;
  name: string;
  email: string;
  createdAt: Date;
};

type GetterName<T> = `get${Capitalize<string & keyof T>}`;
type UserGetters = {
  [K in keyof User as GetterName<User>]: () => User[K];
};
// { getId: () => number; getName: () => string; ... }

// String pattern matching
type ExtractRouteParams<T extends string> =
  T extends `${string}:${infer Param}/${infer Rest}`
    ? Param | ExtractRouteParams<Rest>
    : T extends `${string}:${infer Param}`
      ? Param
    : never;

type Params = ExtractRouteParams<"/users/:userId/posts/:postId">;  // "userId" | "postId"

// CSS units and values
type CssUnit = 'px' | 'em' | 'rem' | '%' | 'vh' | 'vw';
type CssValue = `${number}${CssUnit}`;

let width: CssValue = '100px';    // Valid
let height: CssValue = '50%';     // Valid
// let margin: CssValue = '10';   // Error
// let padding: CssValue = '2ex'; // Error

// API versioning
type ApiVersion = 'v1' | 'v2' | 'v3';
type Endpoint = 'users' | 'products' | 'orders';
type HttpMethod = 'GET' | 'POST' | 'PUT' | 'DELETE';

type ApiUrl = `https://api.example.com/${ApiVersion}/${Endpoint}`;

// Complex example: Dynamic SQL query builder
type Table = 'users' | 'products' | 'orders';
type Column<T extends Table> =
  T extends 'users' ? 'id' | 'name' | 'email' | 'created_at' :
  T extends 'products' ? 'id' | 'name' | 'price' | 'in_stock' :
  T extends 'orders' ? 'id' | 'user_id' | 'total' | 'status' : never;

type WhereCondition<T extends Table> = {
  [K in Column<T>]?: {
    equals?: any;
    notEquals?: any;
    in?: any[];
  };
};

function query<T extends Table>(
  table: T,
  where?: WhereCondition<T>
): `SELECT * FROM ${T}${string}` {
  // Implementation would build the query
  return `SELECT * FROM ${table}` as const;
}

// Usage
const userQuery = query('users', {
  name: { equals: 'John' },
  created_at: { in: ['2023-01-01', '2023-12-31'] }
});
// Type: "SELECT * FROM users WHERE ..."

// Make this file a module so the local EventTarget does not clash with the built-in global one
export {};