JavaScript type checking from react-spring

is.ts

interface Lookup<T = any> {
  [key: string]: T
}
type IsType<U> = <T>(arg: T & any) => arg is Narrow<T, U>
type Narrow<T, U> = [T] extends [Any] ? U : [T] extends [U] ? Extract<T, U> : U

type PlainObject<T> = Exclude<T & Lookup, Function | readonly any[]>

const is = {
  arr: Array.isArray as IsType<readonly any[]>,
  obj: <T extends any>(a: T & any): a is PlainObject<T> =>
    !!a && a.constructor.name === 'Object',
  fun: ((a: unknown) => typeof a === 'function') as IsType<Function>,
  str: (a: unknown): a is string => typeof a === 'string',
  num: (a: unknown): a is number => typeof a === 'number',
  und: (a: unknown): a is undefined => a === undefined,
}