function flattenArray(array) {
  return array.reduce((a, b) => Array.isArray(b) ? a.concat(flattenArray(b)) : a.concat(b), []);
}

// Edit these to change the test parameters.
let numberOfTests = 1000000
let lengthOfArrays = 20;
let maximumNumber = Number.MAX_SAFE_INTEGER

// vvvvvvvvvvvvvvvvvvvvvvvv TESTS vvvvvvvvvvvvvvvvvvvvvvvv

let passed = true;

for (let i = 0; i < numberOfTests; i++) {
  // Create the arrays to test.
  let testArrays = createNestedArray(lengthOfArrays, maximumNumber);

  // Test the flattenArray function.
  let actual = flattenArray(testArrays.nested);

  // Check if the test failed.
  if (!isEqualArrays(testArrays.flattened, actual)) {
    console.log('Tests failed!\n\nExpected:', testArrays.flattened, '\nActual:', actual, '\nTests run:', i + 1);
    passed = false;
    break;
  }
}

// Check if all the tests passed.
if (passed) {
  console.log(numberOfTests + ' tests passed!');
}

// Simple function to check for shallow equality of two arrays.
function isEqualArrays(array1, array2) {
  return array1.every((ele, index) => ele === array2[index]);
}

// The actual testing function that builds both the nested and flattened arrays.
function createNestedArray(length, maxNumber) {
  let outputArray = {
    nested: [],
    flattened: []
  };

  for (let i = 0; i < length; i++) {
    // Generate a random number to use as the next element in the arrays.
    let element = Math.ceil(Math.random() * maxNumber);
    // Generate a random number to select which option to take for the nested array.
    let selector = Math.random();
    // Add the element to the flat array.
    outputArray.flattened.push(element);

    // Short curcuit the loop on the first iteration so that we don't try to put something at the -1 index.
    if (outputArray.nested.length === 0) {
      selector >= 0.5 ? outputArray.nested.push([element]) : outputArray.nested.push(element);
      continue;
    }

    if (selector >= 0.75) {
      // Put the element at the end of the main array but inside its own array.
      outputArray.nested.push([element]);
    } else if (selector >= 0.5) {
      // If the previous element in the main array is an array itself.
      if (Array.isArray(outputArray.nested[outputArray.nested.length - 1])) {
        // Put the element inside that array.
        outputArray.nested[outputArray.nested.length - 1].push(element);
      } else {
        // Put both the previous element and the current element inside an array in place of the previous element.
        outputArray.nested[outputArray.nested.length - 1] = [outputArray.nested[outputArray.nested.length - 1], element];
      }
    } else if (selector >= 0.25) {
      // If the previous element in the main array is an array itself.
      if (Array.isArray(outputArray.nested[outputArray.nested.length - 1])) {
        // Put the element inside that array but in it's own array.
        outputArray.nested[outputArray.nested.length - 1].push([element]);
      } else {
        // Put both the previous element and the current element inside an array in place of the previous element, but put the current element inside it's own array.
        outputArray.nested[outputArray.nested.length - 1] = [outputArray.nested[outputArray.nested.length - 1], [element]];
      }
    } else {
      // Push element onto the end of the main array.
      outputArray.nested.push(element);
    }
  }

  return outputArray;
}