TypeScript Advanced

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What is the resolved type of IsString<number> and IsString<string>?

javascript

type IsString<T> = T extends string ? "yes" : "no";

type A = IsString<number>;
type B = IsString<string>;

ABoth are "yes"

BA is "yes", B is "no"

CBoth are string

DA is "no", B is "yes"

What does the Stringify<T> type produce when applied to Config?

javascript

type Stringify<T> = {
  [K in keyof T]: string;
};

interface Config {
  host: string;
  port: number;
  ssl: boolean;
}

type StringConfig = Stringify<Config>;

A{ host: string; port: number; ssl: boolean } (unchanged)

B{ host: string; port: string; ssl: string }

Cstring

DA union string | number | boolean

What is the type of ReturnType<typeof greet>?

javascript

type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

function greet(name: string): string {
  return `Hello, ${name}`;
}

type R = ReturnType<typeof greet>;

Anever

B(name: string) => string

Cstring

Dany

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