// TypeScript 4.1 Template Literal Types
type HTTPMethod = "GET" | "POST" | "PUT" | "DELETE";
type APIEndpoint = `/api/${string}`;
type APIRoute = `${HTTPMethod} ${APIEndpoint}`;

const route: APIRoute = "GET /api/users"; // Valid

// TypeScript 4.0 Variadic Tuple Types
type Concat<T extends unknown[], U extends unknown[]> = [...T, ...U];

type Result = Concat<[1, 2], [3, 4]>; // [1, 2, 3, 4]

function concat<T extends unknown[], U extends unknown[]>(
  arr1: T, arr2: U
): Concat<T, U> {
  return [...arr1, ...arr2] as Concat<T, U>;
}

// Labeled Tuple Elements
type Address = [streetNumber: number, street: string, city: string];

// Class Property Inference from Constructors
class Point {
  x: number;
  y: number;
  constructor(x: number, y: number) {
    this.x = x;
    this.y = y;
  }
}

// tsconfig.json for project references
{
  "compilerOptions": {
    "composite": true,
    "declaration": true,
    "declarationMap": true
  },
  "references": [
    { "path": "../shared" },
    { "path": "../utils" }
  ]
}

// Modern React with TypeScript
import React, { FC, useState } from 'react';

interface Props {
  initialCount: number;
}

const Counter: FC<Props> = ({ initialCount }) => {
  const [count, setCount] = useState(initialCount);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(c => c + 1)}>Increment</button>
    </div>
  );
};

// Recursive Conditional Types
type Awaited<T> = T extends Promise<infer U> ? Awaited<U> : T;

type DeepReadonly<T> = {
  readonly [P in keyof T]: T[P] extends object
    ? DeepReadonly<T[P]>
    : T[P];
};

// Example usage
type NestedPromise = Promise<Promise<string>>;
type Resolved = Awaited<NestedPromise>; // string

// Result type pattern for error handling
type Result<T, E = Error> =
  | { ok: true; value: T }
  | { ok: false; error: E };

function divide(a: number, b: number): Result<number, string> {
  if (b === 0) {
    return { ok: false, error: "Division by zero" };
  }
  return { ok: true, value: a / b };
}

const result = divide(10, 2);
if (result.ok) {
  console.log(result.value); // TypeScript knows this is number
}

// Strict configuration for production
{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "forceConsistentCasingInFileNames": true
  }
}