TypeScript snippets

compose.ts

// Source: https://medium.com/@minaluke/typescript-compose-function-b7512a7cc012
type ArityOneFn = (arg: any) => any
type PickLastInTuple<T extends any[]> = T extends [
  ...rest: infer U,
  argn: infer L
]
  ? L
  : never
type FirstFnParameterType<T extends any[]> = Parameters<PickLastInTuple<T>>[any]
type LastFnParameterType<T extends any[]> = ReturnType<T[0]>

function compose<
  T extends ArityOneFn[],
  LeftReturnType extends FirstFnParameterType<T>,
  RightReturnType extends LastFnParameterType<T>
>(
  ...fns: T
): (
  // Provide conditional parameter type to support left function
  // without any arguments.
  ...args: LeftReturnType extends never ? never[] : [LeftReturnType]
) => RightReturnType {
  return (...args) => {
    return fns.reduceRight((acc: any, rightFn) => rightFn(acc), args[0])
  }
}