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 right, and a negative n to the left (-length < n < length) :

Array.prototype.rotateRight = function( n ) {   this.unshift( this.splice( n, this.length ) ); } 

Which can then be used this way:

var months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]; months.rotate( new Date().getMonth() ); 

My original version above has a flaw, as pointed out by Christoph in the comments bellow, a correct version is (the additional return allows chaining):

Array.prototype.rotateRight = function( n ) {   this.unshift.apply( this, this.splice( n, this.length ) );   return this; } 

Is there a more compact and/or faster solution, possibly in the context of a JavaScript framework? (none of the proposed versions bellow is either more compact or faster)

Is there any JavaScript framework out there with an array rotate built-in? (Still not answered by anyone)

Answer 1

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([1, 2, 3, 4, 5]);       // [2, 3, 4, 5, 1]; arrayRotate([1, 2, 3, 4, 5], true); // [5, 1, 2, 3, 4]; 

If you need count argument see my other answer:
https://stackoverflow.com/a/33451102 🖤🧡💚💙💜

Answer 2

Type-safe, generic version which mutates the array:

Array.prototype.rotate = (function() {     // save references to array functions to make lookup faster     var push = Array.prototype.push,         splice = Array.prototype.splice;      return function(count) {         var len = this.length >>> 0, // convert to uint             count = count >> 0; // convert to int          // convert count to value in range [0, len)         count = ((count % len) + len) % len;          // use splice.call() instead of this.splice() to make function generic         push.apply(this, splice.call(this, 0, count));         return this;     }; })(); 

In the comments, Jean raised the issue that the code doesn't support overloading of push() and splice(). I don't think this is really useful (see comments), but a quick solution (somewhat of a hack, though) would be to replace the line

push.apply(this, splice.call(this, 0, count)); 

with this one:

(this.push || push).apply(this, (this.splice || splice).call(this, 0, count)); 

---

Using unshift() instead of push() is nearly twice as fast in Opera 10, whereas the differences in FF were negligible; the code:

Array.prototype.rotate = (function() {     var unshift = Array.prototype.unshift,         splice = Array.prototype.splice;      return function(count) {         var len = this.length >>> 0,             count = count >> 0;          unshift.apply(this, splice.call(this, count % len, len));         return this;     }; })();