Flatten an array with n deep levels.

flatten-array.js

const source = [[1, 2, [3, [4]]], 5]

const flatArray = (src) => {
  const flattened = []

  function isArray(value) {
    return value instanceof Array
  }

  // do not modify the original array, instead, clone it
  function flat(arr = [...src], index = 0) {
    const currentValue = arr[index]
    // there are no more elements...
    if (!currentValue) {
      // if there are some inner array yet...
      if (flattened.some(v => v instanceof Array)) {
        const newSrc = [...flattened] // will be our new source array
        flattened.splice(0, flattened.length) // empty the array to start again
        return flat(newSrc, 0)
      }
      // otherwise, return it
      return flattened
    }
    // if the current value is an inner array, just spread it ;)
    if (isArray(currentValue)) {
      flattened.push(...currentValue)
    } else { // else, just add it
      flattened.push(currentValue)
    }
    // next iteration
    return flat(arr, index + 1)
  }

  return flat()
}

const original = source
const flattened = flatArray(source)
console.log('Original array:', original) // [[1, 2, [3, [4]]], 5]
console.log('Flattened array:', flattened) // [1, 2, 3, 4, 5]