# **🦕 Booth: TypeScript (2012)** * **Creator:** Anders Hejlsberg (Microsoft) * **Design Philosophy:** Static types that scale. TypeScript is a strict syntactical superset of JavaScript, meaning any valid JavaScript is valid TypeScript. It builds powerful static compilation analysis over JavaScript runtimes without any runtime performance penalty. * **Key Innovations:** Structural static type checking, generic constraints, mapped and template literal types, and type inference. ## **📐 Structural Subtyping Algebra** Unlike languages with nominal type systems (like Java or C++), TypeScript compares object compatibility strictly structurally. We define structural subtyping for two types ![][image1] and ![][image2]: ![][image3]An instance of type ![][image1] can be assigned to a variable of type ![][image2] if it has at least all the properties of ![][image2], regardless of whether ![][image1] explicitly implements ![][image2]. ## **🚀 Hello, World\!** const greeting: string \= "Hello, World\!"; console.log(greeting); ## **🎨 Showpiece: Discriminated Unions & Readonly Interfaces** This showcase models strict algebraic type-safety using discriminated union structures and mapped keys evaluated entirely at compile-time. // Define structured interfaces interface SuccessPayload { type: "success"; // Discriminant field data: string; } interface ErrorPayload { type: "error"; // Discriminant field errorCode: number; } // Combine models into an Algebraic Type type ApiResponse \= SuccessPayload | ErrorPayload; // A generic utility that turns all keys of a type into Readonly values at compile-time type SafePayload\ \= { readonly \[P in keyof T\]: T\[P\]; }; function handleResponse(response: SafePayload\): string { // The compiler enforces static verification based on the discriminant string switch (response.type) { case "success": return \`Success\! Data received: "${response.data.toUpperCase()}"\`; case "error": return \`System Error. Code reference: \[${response.errorCode}\]\`; default: const exhaustiveCheck: never \= response; // Enforces completeness return exhaustiveCheck; } } // Client testing const okResponse: ApiResponse \= { type: "success", data: "Data loaded from API server" }; const errResponse: ApiResponse \= { type: "error", errorCode: 503 }; console.log(handleResponse(okResponse)); console.log(handleResponse(errResponse)); [image1]: [image2]: [image3]: