Flatten an array

flatten.js

const nestedArray = [[1,2,[3]],4]

const flattened = flatten(nestedArray)

const expected = [1, 2, 3, 4]

/**
 * Because I didn't want to pull in a test framework
 * for this case, I am just writing a custom assertion
 * to verify that the array is flattened as expected.
 */
if (!arraysEqual(expected, flattened)) {
  console.log('Failed asserting that these 2 arrays are identicals: ', expected, flattened)
  throw Error()
} else {
  console.log('It is flattened')
}

/**
 * Reduce the array recursively into a flat array.
 */
function flatten(arr) {
  return arr.reduce((flattened, item) => {
    const other = item instanceof Array ? flatten(item) : [item]
    return [...flattened, ...other]
  }, [])
}

/**
 * Compare two arrays with strict comparison
 * of all the items.
 */
function arraysEqual(first, second) {
  if(first.length !== second.length) {
    return false
  }
  for(let i = first.length; i--;) {
      if(first[i] !== second[i])
          return false;
  }
  return true;
}