Rotate the elements in an array in JavaScript

Question

I was wondering what was the most efficient way to rotate a JavaScript array.

I came up with this solution, where a positive n rotates the array to the

Answer 1

see http://jsperf.com/js-rotate-array/8

function reverse(a, from, to) {
  --from;
  while (++from < --to) {
    var tmp = a[from];
    a[from] = a[to];
    a[to] = tmp;
  }
}

function rotate(a, from, to, k) {
  var n = to - from;
  k = (k % n + n) % n;
  if (k > 0) {
    reverse(a, from, from + k);
    reverse(a, from + k, to);
    reverse(a, from, to);
  }
}

Answer 2

Not sure about the efficiency but I would do it in this non mutating way :

	Array.prototype.rotate = function( n ) {
  
		 return this.map( (item, index)=> this[ (this.length + index + n)%this.length ] )
	}

Answer 3

You can use push(), pop(), shift() and unshift() methods:

function arrayRotate(arr, reverse) {
  if (reverse) arr.unshift(arr.pop());
  else arr.push(arr.shift());
  return arr;
}

usage:

arrayRotate(['h','e','l','l','o']);       // ['e','l','l','o','h'];
arrayRotate(['h','e','l','l','o'], true); // ['o','h','e','l','l'];

If you need count argument see my other answer: https://stackoverflow.com/a/33451102

Answer 4

So many of these answers seem over-complicated and difficult to read. I don't think I saw anyone using splice with concat...

function rotateCalendar(){
    var cal=["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],
    cal=cal.concat(cal.splice(0,new Date().getMonth()));
    console.log(cal);  // return cal;
}

---

console.log outputs (*generated in May):

["May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec", "Jan", "Feb", "Mar", "Apr"]

---

As for compactness, I can offer a couple of generic one-liner functions (not counting the console.log | return portion). Just feed it the array and the target value in the arguments.

I combine these functions into one for a four-player card game program where the array is ['N','E','S','W']. I left them separate in case anyone wants to copy/paste for their needs. For my purposes, I use the functions when seeking whose turn is next to play/act during different phases of the game (Pinochle). I haven't bothered testing for speed, so if someone else wants to, feel free to let me know the results.

*notice, the only difference between functions is the "+1".

function rotateToFirst(arr,val){  // val is Trump Declarer's seat, first to play
    arr=arr.concat(arr.splice(0,arr.indexOf(val)));
    console.log(arr); // return arr;
}
function rotateToLast(arr,val){  // val is Dealer's seat, last to bid
    arr=arr.concat(arr.splice(0,arr.indexOf(val)+1));
    console.log(arr); // return arr;
}

---

combination function...

function rotateArray(arr,val,pos){
    // set pos to 0 if moving val to first position, or 1 for last position
    arr=arr.concat(arr.splice(0,arr.indexOf(val)+pos));
    return arr;
}
var adjustedArray=rotateArray(['N','E','S','W'],'S',1);

adjustedArray=

W,N,E,S

Answer 5

How about incrementing a counter and then getting the remainder of a division by the array length to get where you are supposed to be.

var i = 0;
while (true);
{
    var position = i % months.length;
    alert(months[position]);
    ++i;
}

Language syntax aside this should work fine.

Answer 6

** Using Latest version of JS we can build it every easily **

 Array.prototype.rotateLeft = function (n) {
   this.unshift(...this.splice(-(n), n));
    return this
  }

here moves: number of rotations ,a Array that you can pass random number

let a = [1, 2, 3, 4, 5, 6, 7];
let moves = 4;
let output = a.rotateLeft(moves);
console.log("Result:", output)