TypeScript

TypeScript -> Transpiler (Babel, swc, or Sucrase) -> JavaScript

Union types and type guards

Key Patterns Summary:

1. Discriminated Unions (Most Important!)

type Response = 
  | { status: "success"; data: any }
  | { status: "error"; message: string }

// TypeScript narrows based on 'status'

2. Type Guard Operators

typeof x === "string"    // Primitives
x instanceof ClassName   // Class instances
"property" in x          // Property existence
Array.isArray(x)         // Arrays

3. Custom Type Guards

function isType(x: unknown): x is Type {
  return /* check logic */;
}