Flatten some arrays

flatten.js

/** 
 * Yo dawg I heard you like arrays, so if you put arrays in your
 * arrays here's a function that can flatten it. O(n) complexity
 * 
 *    e.g flattenArray([[1,2,[3]],4]) -> [1,2,3,4]
 * 
 * @param {Array} inputArray - Unflattened array
 * @returns {Array} flattened array, bye bye pesky nested arrays
 */
function flattenArray(inputArray, outputArray = []) {
  for (let i = 0; i < inputArray.length; i++) {
    const inputValue = inputArray[i];
    if (Array.isArray(inputValue))  flattenArray(inputValue, outputArray);
    else outputArray.push(inputValue);
  }
  return outputArray;
}

test-flatten.js

function testFlatten() {
  const flattenTestCases = [{
    inputArray: [1],
    expectedOutput: [1],
  }, {
    inputArray: [1, 2, 3, 4],
    expectedOutput: [1, 2, 3, 4],
  }, {
    inputArray: [[1,2,[3]],4],
    expectedOutput: [1, 2, 3, 4],
  }, {
    inputArray: [[1,[2],3],4],
    expectedOutput: [1, 2, 3, 4],
  }, {
    inputArray: [[1,2,[3]],4, [6], [8, [9], 10], 11], 
    expectedOutput:  [1, 2, 3, 4, 6, 8, 9, 10, 11],
  }]

  for (let testCase of flattenTestCases) {
    const outputArray = flattenArray(testCase.inputArray);
    if (!areArraysEqual(outputArray, testCase.expectedOutput)) {
      console.error(`Arrays are inequal:\n\n\tExpected: ${testCase.expectedOutput}\n\tActual: ${outputArray}`);
      return;
    }
  }
  console.log('flatten is humming long fine');
}

function areArraysEqual(arr1 = [], arr2 = []) {
  // arrays cant be equal if their lengths aren't equal
  if (arr1.length !== arr2.length) return false;
  
  // compare each index in each array
  for (let i = 0; i < arr1.length; i++) {
    if (arr1[i] !== arr2[i]) return false;
  }

  // checks pass! arrays are equal
  return true;
}