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

I would probably do something like this:

Array.prototype.rotate = function(n) {
    return this.slice(n, this.length).concat(this.slice(0, n));
}

---

Edit    Here’s a mutator version:

Array.prototype.rotate = function(n) {
    while (this.length && n < 0) n += this.length;
    this.push.apply(this, this.splice(0, n));
    return this;
}

Answer 2

Here is a very simple way to shift items in an array:

function rotate(array, stepsToShift) {

    for (var i = 0; i < stepsToShift; i++) {
        array.unshift(array.pop());
    }

    return array;
}

Answer 3

When I couldn't find a ready-made snippet to start a list of days with 'today', I did it like this (not quite generic, probably far less refined than the above examples, but did the job):

//returns 7 day names with today first
function startday() {
    const days = ['Sun','Mon','Tue','Wed','Thu','Fri','Sat'];
    let today = new Date();
    let start = today.getDay(); //gets day number
    if (start == 0) { //if Sunday, days are in order
        return days
    }
    else { //if not Sunday, start days with today
        return days.slice(start).concat(days.slice(0,start))
    }
}

Thanks to a little refactor by a better programmer than me it's a line or two shorter than my initial attempt, but any further comments on efficiency are welcome.

Answer 4

Array in JS has below built in method which can be used to rotate an array quite easily and obviously these methods are immutable in nature.

• push: Inserts the item to end of the array.
• pop: Removes the item from the end of the array.
• unshift: Inserts the item to the beginning of the array.
• shift: Removes the item from the beginning of the array.

The below solution (ES6) takes two arguments , array needs to be rotated and n , number of times the array should be rotated.

const rotateArray = (arr, n) => {
  while(arr.length && n--) {
    arr.unshift(arr.pop());
  }
  return arr;
}

rotateArray(['stack', 'overflow', 'is', 'Awesome'], 2) 
// ["is", "Awesome", "stack", "overflow"]

It can be added to Array.prototype and can be used all across your application

Array.prototype.rotate = function(n) {
 while(this.length && n--) {
   this.unshift(this.pop());
 }
 return this;
}
[1,2,3,4].rotate(3); //[2, 3, 4, 1]

Answer 5

Using ES6's spread for an immutable example ...

[...array.slice(1, array.length), array[0]]

and

[array[array.items.length -1], ...array.slice(0, array.length -1)]

It's probably not the most efficient though but it's concise.

Answer 6

Follow a simpler approach of running a loop to n numbers and shifting places upto that element.

function arrayRotateOne(arr, n) {
  for (let i = 0; i < n; i++) {
    arr.unshift(arr.pop());
  }
  return arr;
}
console.log( arrayRotateOne([1,2,3,4,5,6],2));



function arrayRotateOne(arr,n) {
  for(let i=0; i<n;i++){
      arr.push(arr.shift());
      console.log('execute',arr)
    }
     return arr;
 }

console.log( arrayRotateOne([1,2,3,4,5,6],2));