Extract methods/function properties from a type in TypeScript

methods-of.ts

type AnyFunction = (...args: any[]) => any;

type FunctionPropertyOf<T> = {
  [P in keyof T]: T[P] extends AnyFunction ? P : never;
}[keyof T];

type MethodOf<T> = {
  [P in keyof T]: T[P] extends (this: ThisType<T[P]>, ...args: any[]) => any ? P : never;
}[keyof T];


type Foo = MethodOf<RemoveIndexSignature<Storage>> // "clear" | "getItem" | "key" | "removeItem" | "setItem"
type Bar = FunctionPropertyOf<RemoveIndexSignature<Storage>> // "clear" | "getItem" | "key" | "removeItem" | "setItem"

/**
 * https://stackoverflow.com/a/66252656/10325032
 */
type RemoveIndexSignature<T> = {
  [P in keyof T as string extends P ? never : number extends P ? never : P]: T[P];
};