const a = [[1,2,[3]],4]

​

const flattenArray = input => {

  if (!Array.isArray(input)) {

    throw new Error("Input must be an array")

  }

  let result = []

  for (let element of input) {

    if (Array.isArray(element)) {

      result = result.concat(flattenArray(element))

    } else {

      result.push(element)

    }

  }

  return result

}

​

console.log(flattenArray(a))