Set your Publisher ID in src/config/ads.ts to display live ads.
Conditional & Mapped Types
Building dynamic type transformers with infer and keyof mappings
Conditional types take the form T extends U ? X : Y. When combined with mapped types and the infer keyword, you can build powerful meta-programming type machinery.
Understanding Conditional and Mapped Types
Conditional types: Express logic at compile time: type IsString<T> = T extends string ? true : false;.
The infer keyword: Allows you to deduce and extract a type inside a conditional branch (e.g., unwrapping Promises: type Awaited<T> = T extends Promise<infer R> ? R : T;).
Mapped types: Iterate over keys using [K in keyof T] to transform property attributes or names dynamically.
Template literal types: Combine string unions to produce patterns: type Event = ${string}_event;.
// Custom type helper that unwraps Promise return values
type UnwrapPromise<T> = T extends Promise<infer U> ? U : T;
// Async fetch function signature
async function fetchMetrics() {
return { activeUsers: 4820, latencyMs: 14.2 };
}
// Extract the resolved return type automatically
type Metrics = UnwrapPromise<ReturnType<typeof fetchMetrics>>;
const currentMetrics: Metrics = {
activeUsers: 5100,
latencyMs: 11.8
};
console.log(`Users: ${currentMetrics.activeUsers}`);Set your Publisher ID in src/config/ads.ts to display live ads.
Test Yourself With Exercise
Which TypeScript keyword allows you to introduce and extract a type variable inside a conditional type check?