// 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]

function flatten (arr) {
  let result = arr
// check if arr contains an array
  while(result.find(item => Array.isArray(item))) {
    // merge arr w/ top level arr
    result = [].concat.apply([], result)
  }
  // when complete return flattened arr
  return result
}

function testFlatten () {
  let assertions = [
    {
      input: [ [1,2,[3, [4, 5]]], 6],
      output: [1,2,3,4,5,6],
      desc: 'test 3 level array'
    },
    {
      input: [[1,2,[3]],4],
      output: [1,2,3,4],
      desc: 'test 2 level array'
      
    },
  ]
  assertions.forEach(assertion => {
    if (!testArrayEquality(flatten(assertion.input), assertion.output)) { 
      console.warn(`Test failed - ${assertion.desc}`)
    }
    else {
      console.log(`Test passed - ${assertion.desc}`)
    }
  })
}

function testArrayEquality (src, comparison) {
  return src.length === comparison.length && src.every((item, index) => {
    return item === comparison[index]; 
  })
}

testFlatten()