Flatten Nested Arrays (ES6)

// Create our array-flattening method (using a recursive approach)
const flatten = (arr) => {

  // Return a new flattened array
  return arr.reduce((prev, next) => {
    
    // Make sure we have an array to add items to
    prev = Array.isArray(prev) ? prev : [prev]
    
    // If the next element in `arr` is not an array, add it to our flattened array
    // Otherwise, pass the child array to this method, to be flattened recursively
    return prev.concat(Array.isArray(next) ? flatten(next) : next)
  })
}

// Try it out!
const x = [1, [2, 3], [[4], 5, 6]]
const y = flatten(x) // [1, 2, 3, 4, 5, 6]