Recursive Array Flattener

flatten.js

// Recursive function that loops through each array, adding it's contents to the original until we're out of numbers.
const flatten = (integersArray, previousArray) => {
  // First time it's called there is no second argument, so make that an empty array.
  const newArray = previousArray || []
  // integersArray needs elements or just return previous and exit recursion.
  if (integersArray && integersArray.length > 0) {
    integersArray.forEach(i => {
      // if it's an integer, add it to the returned array.
      if (typeof i === "number") {
        newArray.push(i)
        // If not it's an Array, so dive into it, passing our working array.
      } else if (i instanceof Array) {
        flatten(i, newArray)
      }
    })
  }
  return newArray
}

// Arrays need to be converted to a string to be comparable.
const prepForComparison = a => a.join(",")

// Simple DIY testing function
const testFlatten = (testArray, expectedArray) => {
  const result = prepForComparison(flatten(testArray))
  const expectedResult = prepForComparison(expectedArray)
  if (result === expectedResult) {
    console.log(`Test Passed! [${result}] does equal [${expectedArray}]`)
  } else {
    console.log(`Test Failed! [${result}] does NOT equal [${expectedArray}]`)
  }
}

// Run the tests by passing what the array should look like as 2nd argument.
testFlatten([1, 2, [3, 4]], [1, 2, 3, 4])
testFlatten([1, 2, [3, 4], 5, [6]], [1, 2, 3, 4, 5, 6])
testFlatten([1, 2, [3, 4], 5, [6]], [1, 2, 3, 4, 5])  // Failing test, just to be complete.