vargs node variable number of arguments with a callback

varg.js

  const varg = (fn, arity) => (...args) => {
    const l = args.length < arity ? args.length : arity;
    return typeof args[l - 1] === 'function'
      ? fn(
          ...args.slice(0, -1),
          ...new Array(arity - l).fill(undefined),
          args[l - 1]
        )
      : fn(...args, ...new Array(arity - args.length).fill(undefined));
  };