Flatten an array of arbitrarily nested arrays of integers into a flat array of integers

flatten-integers.js

// flatten an array of arbitrarily nested arrays of integers
// into a flat array of integers.
// e.g. [[1,2,[3]],4] -> [1,2,3,4]
//
// could be extended to check for objects, etc. but spec says to only 
// expect integers.

const flattenIntegers = (array, flattenedArray = []) => {
  array.forEach(element => {
    if (Array.isArray(element)) {
      flattenIntegers(element, flattenedArray)
    } else {
      flattenedArray.push(element)
    }
  })
  
  return flattenedArray
}

// Eyeball testing (would set up test suite with Mocha for this with variety of input)
const inputArray = [[1,2,[3]],4,[[[[[[[5],6]]]]]]]

const flattened = flattenIntegers(inputArray)

console.log(inputArray)
console.log(flattened)