Javascript flatten similar to ruby's flatten

flatten.js

function flatten(arr, depth = Infinity, result = []) {
  if(!Array.isArray(arr)) return null
  for (const value of arr) {
    if (depth > 0 && Array.isArray(value)) {
      if (depth > 1) {
        flatten(value, depth - 1, result)
      } else {
        result.push(...value)
      }
    } else {
      result[result.length] = value
    }
  }
  return result
}


// Tests
const superFlat = JSON.stringify(flatten( [[ [[[[1]]]], 2, [ 3 ] ], 4] )) === JSON.stringify([1, 2, 3, 4])
if ( !superFlat ) throw 'Failed to flatten many layers deep'

const oneLayer = JSON.stringify(flatten( [1,2,3,[[4]]], 1 )) === JSON.stringify([1,2,3,[4]])
if (!oneLayer) throw 'Failed to only flatten one layer'

const crazyInput = JSON.stringify(flatten( {1:1, 2:2, 3:3} )) === JSON.stringify(null)
if(!crazyInput) throw 'Does not handle crazy inputs'

console.log('Everything flattened successfully')