flatten-array.js

const flatten = (() => {
  function flatten(arr, list) {
    if (Array.isArray(arr)) {
      arr.forEach(item => flatten(item, list))
    } else {
      list.push(arr)
    }
    
    return list
  }
  
  return (arr) => {
    const list = []
    
    return flatten(arr, list)
  }
})()

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

console.log(
  flatten([1, 2, [3, 4, [5, 6]]])  
)