flatten a nested array

flat.js

const isArr = arg => Object.prototype.toString.call(arg) === '[object Array]'
const flat = inputAny =>
  (
    isArr(inputAny[0])
      ? flat(inputAny[0])
      : [inputAny[0]]
  )
  .concat(
    inputAny.length > 1
      ? flat(inputAny.slice(1))
      : []
  )
console.log(flat([0, [1, [[2, 3]], 4]]))

y_combinator_flat.js

const Y = f => (x => f(x(x)))(x => f((...y) => x(x)(...y)))

const isArr = arg => Object.prototype.toString.call(arg) === '[object Array]'

const flat = Y(
  f => inputAny =>
    (
      isArr(inputAny[0])
        ? f(inputAny[0])
        : [inputAny[0]]
    )
    .concat(
      inputAny.length > 1
        ? f(inputAny.slice(1))
        : []
    )
)

console.log(flat([0, [1, [[2, 3]], 4]]))