JS Array permutation

other_type_of_permutation.js

function permute(arr) {
  if (arr.length === 2) {
    return [arr.slice(), arr.slice().reverse()]
  }

  return arr.map(function(element, index, array) {
    var new_array = array.slice()
    var last_element = new_array.splice(index, 1)
    return permute(new_array).map(function(x) {
      return last_element.concat(x)
    })
  }).reduce(function(a, b) {
    return a.concat(b)
  })
}

Array.permute = permute
Array.prototype.permute = function() {
    return Array.permute(this)
}

permute.js

function permute(arr) {
  if (arr.length === 2) {
    return [arr.slice(), arr.slice().reverse()]
  }

  return [].concat.apply([], arr.map(function(element, index, array) {
    var new_array = array.slice()
    var last_element = new_array.splice(index, 1)
    return Array.prototype.map.apply(permute(new_array), [
      function(x) {
        return last_element.concat(x)
      }
    ])
  }))
}

Array.permute = permute
Array.prototype.permute = function() {
    return Array.permute(this)
}