Example of how to flatten a javascript array with no dependencies

flat.js

// Run with browser devtools or nodejs

function flat (array) {
  var out = []
  array || (array = [])

  function recur (internalArray) {
    for (var i=0; i<internalArray.length; i++) {
      var item = internalArray[i]
      if (Array.isArray(item)) {
        recur(item)
      } else {
        out.push(item)
      }
    }
  }

  recur(array)
  return out
}

// Mini test framework so no deps used
function test (description, input, expectFn) {
  var result
  try { result = flat(input) } catch(e) { result = e }
  var expectedResult = expectFn(result, input);
  if (!expectedResult) {
    console.error('Test failed: ', description, ' got ', result)
  } else {
    console.log('Test passed: ', description)
  }
}

function isSameArray (arrayA, arrayB) {
  return (arrayA.length === arrayB.length && arrayA.every((v,i) => v === arrayB[i]))
}

test('null returns empty array', null, function(result) {
  return (result.length === 0) ? true : false
})

test('undefined returns empty array', undefined, function(result) {
  return (result.length === 0) ? true : false
})

test('empty array returns empty array', [], function(result) {
  return (result.length === 0) ? true : false
})

test('flat array returns flat array', [1, 2, 3], function(result, input) {
  return isSameArray(result, input)
})

test('nested array returns nested array', [[1,2,[3]],4], function(result, input) {
  return isSameArray(result, [1, 2, 3, 4])
})

test('nested null array returns nested array', [[null,null,[]],null], function(result, input) {
  return isSameArray(result, [null,null,null])
})

test('nested empty arrays returns empty array', [[[],[],[[[[]]]]],[]], function(result, input) {
  return isSameArray(result, [])
})