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

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;
    };
})();