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HTML / CSS / JS

// express-extension.d.ts — augment Express Request
import "express";
declare module "express-serve-static-core" {
    interface Request {
        user?: { id: number; email: string; role: string };
        requestId: string;
    }
}

// Now inside a middleware or route handler:
// req.user?.email  — TypeScript knows about user

// global.d.ts — augment global Window
declare global {
    interface Window {
        __APP_VERSION__: string;
        __FEATURE_FLAGS__: Record<string, boolean>;
    }

// Augment process.env for Node.js
    namespace NodeJS {
        interface ProcessEnv {
            NODE_ENV: "development" | "production" | "test";
            DATABASE_URL: string;
            JWT_SECRET: string;
            PORT?: string;
        }
    }
}

export {}; // ensures file is treated as a module

// ambient.d.ts — describe a JS library without types
declare module "legacy-utils" {
    export function formatCurrency(
        amount: number,
        currency: string,
        locale?: string
    ): string;

export function slugify(text: string): string;

export const VERSION: string;
}

// Using the ambient declaration
import { formatCurrency, slugify } from "legacy-utils";
const price = formatCurrency(19.99, "USD", "en-US");
const slug  = slugify("Hello World"); // "hello-world"