This is the official WCA Cube Scrambling JavaScript source, taken from http://www.jaapsch.net/scramble_cube.htm.

cube.js

var size=3;
var seqlen=30;
var numcub=5;
var mult=false;
var cubeorient=false;
var colorString = "yobwrg";  //In dlburf order. May use any colours in colorList below

// list of available colours
var colorList=new Array(
	'y', "yellow", "yellow",
	'b', "blue",   "blue",
	'r', "red",    "red",
	'w', "white",  "white",
	'g', "green",  "green",
	'o', "#ff8000","orange",   // 'orange' is not an official html colour name
	'p', "purple", "purple",
	'0', "gray",   "grey"      // used for unrecognised letters, or when zero used.
);

var colors=new Array(); //stores colours used
var seq=new Array();	// move sequences
var posit = new Array();	// facelet array
var flat2posit;	//lookup table for drawing cube
var colorPerm = new Array(); //dlburf
colorPerm[ 0] = new Array(0,1,2,3,4,5);
colorPerm[ 1] = new Array(0,2,4,3,5,1);
colorPerm[ 2] = new Array(0,4,5,3,1,2);
colorPerm[ 3] = new Array(0,5,1,3,2,4);
colorPerm[ 4] = new Array(1,0,5,4,3,2);
colorPerm[ 5] = new Array(1,2,0,4,5,3);
colorPerm[ 6] = new Array(1,3,2,4,0,5);
colorPerm[ 7] = new Array(1,5,3,4,2,0);
colorPerm[ 8] = new Array(2,0,1,5,3,4);
colorPerm[ 9] = new Array(2,1,3,5,4,0);
colorPerm[10] = new Array(2,3,4,5,0,1);
colorPerm[11] = new Array(2,4,0,5,1,3);
colorPerm[12] = new Array(3,1,5,0,4,2);
colorPerm[13] = new Array(3,2,1,0,5,4);
colorPerm[14] = new Array(3,4,2,0,1,5);
colorPerm[15] = new Array(3,5,4,0,2,1);
colorPerm[16] = new Array(4,0,2,1,3,5);
colorPerm[17] = new Array(4,2,3,1,5,0);
colorPerm[18] = new Array(4,3,5,1,0,2);
colorPerm[19] = new Array(4,5,0,1,2,3);
colorPerm[20] = new Array(5,0,4,2,3,1);
colorPerm[21] = new Array(5,1,0,2,4,3);
colorPerm[22] = new Array(5,3,1,2,0,4);
colorPerm[23] = new Array(5,4,3,2,1,0);

function parse() {
	var s="";
	var urlquery=location.href.split("?")
	if(urlquery.length>1){
		var urlterms=urlquery[1].split("&")
		for( var i=0; i<urlterms.length; i++){
			var urllr=urlterms[i].split("=");
			if(urllr[0]=="size") {
				if(urllr[1]-0 >= 2 ) size=urllr[1]-0;
			} else if(urllr[0]=="len") {
				if(urllr[1]-0 >= 1 ) seqlen=urllr[1]-0;
			} else if(urllr[0]=="num"){
				if(urllr[1]-0 >= 1 ) numcub=urllr[1]-0;
			} else if(urllr[0]=="multi") {
				mult=(urllr[1]=="on");
			} else if(urllr[0]=="cubori") {
				cubeorient=(urllr[1]=="on");
			} else if(urllr[0]=="col") {
				if(urllr[1].length==6) colorString = urllr[1];
			}
		}
	}

	// expand colour string into 6 actual html color names
	for(var k=0; k<6; k++){
		colors[k]=colorList.length-3;	// gray
		for( var i=0; i<colorList.length; i+=3 ){
			if( colorString.charAt(k)==colorList[i] ){
				colors[k]=i;
				break;
			}
		}
	}
}
parse();

function scramble(){
	//tl=number of allowed moves (twistable layers) on axis -- middle layer ignored
	var tl=size; if(mult || (size&1)!=0 ) tl--;
	//set up bookkeeping
	var axsl=new Array(tl); // movement of each slice
	var axam=new Array(0,0,0); // number of slices moved each amount
	var la; // last axis moved

	// for each cube scramble
	for( n=0; n<numcub; n++){
		// initialise this scramble
		la=-1;
		seq[n]=new Array(); // moves generated so far

		// while generated sequence not long enough
		while( seq[n].length<seqlen ){

			// choose a different axis than previous one
			var ax;
			do{
				ax=Math.floor(Math.random()*3);
			}while( ax==la );

			// reset slice/direction counters
			for( var i=0; i<tl; i++) axsl[i]=0;
			axam[0]=axam[1]=axam[2]=0;
			var moved = 0;

			// generate moves on this axis
			do{
				// choose random unmoved slice
				var sl;
				do{
					sl=Math.floor(Math.random()*tl);
				}while( axsl[sl]!=0 );
				// choose random amount
				var q=Math.floor(Math.random()*3);

				if( mult // multislice moves have no reductions so always ok
					|| tl!=size // odd cube always ok since middle layer is reference
					||   (axam[q]+1)*2<tl // less than half the slices in same direction also ok
					|| ( (axam[q]+1)*2==tl && axam[0]+axam[1]+axam[2]-axam[q]==0 ) // exactly half the slices move in same direction and no other slice moved
				){
					axam[q]++;// adjust direction count
					moved++;
					axsl[sl]=q+1;// mark the slice has moved amount
				}
			}while( Math.floor(Math.random()*3)==0 // 2/3 prob for other axis next,
					&& moved<tl		// must change if all layers moved
					&& moved+seq[n].length<seqlen ); // must change if done enough moves

			// append these moves to current sequence in order
			for( var sl=0; sl<tl; sl++){
				if( axsl[sl] ){
					var q=axsl[sl]-1;

					// get semi-axis of this move
					var sa = ax;
					var m = sl;
					if(sl+sl+1>=tl){ // if on other half of this axis
						sa+=3; // get semi-axis (i.e. face)
						m=tl-1-m; // slice number counting from that face
						q=2-q; // opposite direction when looking at that face
					}
					// store move
					seq[n][seq[n].length]=(m*6+sa)*4+q;
				}
			}

			// avoid this axis next time
			la=ax;
		}

		// do a random cube orientation if necessary
		seq[n][seq[n].length]= cubeorient ? Math.floor(Math.random()*24) : 0;
	}

	// build lookup table
	flat2posit=new Array(12*size*size);
	for(i=0; i<flat2posit.length; i++) flat2posit[i]=-1;
	for(i=0; i<size; i++){
		for(j=0; j<size; j++){
			flat2posit[4*size*(3*size-i-1)+  size+j  ]=        i *size+j;	//D
			flat2posit[4*size*(  size+i  )+  size-j-1]=(  size+i)*size+j;	//L
			flat2posit[4*size*(  size+i  )+4*size-j-1]=(2*size+i)*size+j;	//B
			flat2posit[4*size*(       i  )+  size+j  ]=(3*size+i)*size+j;	//U
			flat2posit[4*size*(  size+i  )+2*size+j  ]=(4*size+i)*size+j;	//R
			flat2posit[4*size*(  size+i  )+  size+j  ]=(5*size+i)*size+j;	//F
		}
	}

/*
       19                32
   16           48           35
       31   60      51   44
   28     80    63    67     47
              83  64
          92          79
              95  76

                 0
             12     3
                15
*/
}

function scramblestring(n){
	var s="",j;
	for(var i=0; i<seq[n].length-1; i++){
		if( i!=0 ) s+=" ";
		var k=seq[n][i]>>2;
		if(size<=5){
			s+="DLBURFdlburf".charAt(k);
		}else{
			j=k%6; k=(k-j)/6;
			s+="DLBURF".charAt(j);
			if(k) s+="<sub>"+(k+1)+"<\/sub>";
		}
		j=seq[n][i]&3;
		if(j!=0) s+=" 2'".charAt(j);
	}

	// add cube orientation
	if( cubeorient ){
		var ori = seq[n][seq[n].length-1];
		s="Top:"+colorList[ 2+colors[colorPerm[ori][3]] ]
			+"&nbsp;&nbsp;&nbsp;Front:"+colorList[2+ colors[colorPerm[ori][5]] ]+"<br>"+s;
	}
	return s;
}

function imagestring(nr){
	var s="",i,f,d=0,q;

	// initialise colours
	for( i=0; i<6; i++)
		for( f=0; f<size*size; f++)
			posit[d++]=i;

	// do move sequence
	for(i=0; i<seq[nr].length-1; i++){
		q=seq[nr][i]&3;
		f=seq[nr][i]>>2;
		d=0;
		while(f>5) { f-=6; d++; }
		do{
			doslice(f,d,q+1);
			d--;
		}while( mult && d>=0 );
	}

	// build string containing cube
	var ori = seq[nr][seq[nr].length-1];
	d=0;
	s="<table border=1 cellpadding=0 cellspacing=0>";
	for(i=0;i<3*size;i++){
		s+="<tr>";
		for(f=0;f<4*size;f++){
			if(flat2posit[d]<0){
				s+="<td><\/td>";
			}else{
				var c = colorPerm[ori][posit[flat2posit[d]]];
				s+="<td bgcolor="+colorList[colors[c]+1]+"><img src='images/blank.gif' width=10 height=10><\/td>";
			}
			d++;
		}
		s+="<\/tr>";
	}
	s+="<\/table>";
	return(s);
}

function doslice(f,d,q){
	//do move of face f, layer d, q quarter turns
	var f1,f2,f3,f4;
	var s2=size*size;
	var c,i,j,k;
	if(f>5)f-=6;
	// cycle the side facelets
	for(k=0; k<q; k++){
		for(i=0; i<size; i++){
			if(f==0){
				f1=6*s2-size*d-size+i;
				f2=2*s2-size*d-1-i;
				f3=3*s2-size*d-1-i;
				f4=5*s2-size*d-size+i;
			}else if(f==1){
				f1=3*s2+d+size*i;
				f2=3*s2+d-size*(i+1);
				f3=  s2+d-size*(i+1);
				f4=5*s2+d+size*i;
			}else if(f==2){
				f1=3*s2+d*size+i;
				f2=4*s2+size-1-d+size*i;
				f3=	 d*size+size-1-i;
				f4=2*s2-1-d-size*i;
			}else if(f==3){
				f1=4*s2+d*size+size-1-i;
				f2=2*s2+d*size+i;
				f3=  s2+d*size+i;
				f4=5*s2+d*size+size-1-i;
			}else if(f==4){
				f1=6*s2-1-d-size*i;
				f2=size-1-d+size*i;
				f3=2*s2+size-1-d+size*i;
				f4=4*s2-1-d-size*i;
			}else if(f==5){
				f1=4*s2-size-d*size+i;
				f2=2*s2-size+d-size*i;
				f3=s2-1-d*size-i;
				f4=4*s2+d+size*i;
			}
			c=posit[f1];
			posit[f1]=posit[f2];
			posit[f2]=posit[f3];
			posit[f3]=posit[f4];
			posit[f4]=c;
		}

		/* turn face */
		if(d==0){
			for(i=0; i+i<size; i++){
				for(j=0; j+j<size-1; j++){
					f1=f*s2+         i+         j*size;
					f3=f*s2+(size-1-i)+(size-1-j)*size;
					if(f<3){
						f2=f*s2+(size-1-j)+         i*size;
						f4=f*s2+         j+(size-1-i)*size;
					}else{
						f4=f*s2+(size-1-j)+         i*size;
						f2=f*s2+         j+(size-1-i)*size;
					}
					c=posit[f1];
					posit[f1]=posit[f2];
					posit[f2]=posit[f3];
					posit[f3]=posit[f4];
					posit[f4]=c;
				}
			}
		}
	}
}

function help(){
	alert("Cube Scrambler\n\n"+
		"This cube scrambler can scramble a cube of any size.\n"+
		"Enter the cube size, the number of scrambles you want,\n"+
		"and the length of each scramble (in htm). If you then\n"+
		"press the Scramble button the page will reload and show\n"+
		"the new scrambles. Every time you then reload the page\n"+
		"or click the button, a new set of scrambles is generated.\n\n"+

		"Scrambles:\n"+
		"The scrambles will not contain any moves that cancel each\n"+
		"other, nor moves that simplify to a cube rotation.\n\n"+

		"Notation:\n"+
		"Standard FLUBRD notation is used for the 2x2x2 and\n"+
		"3x3x3 cubes. With 4x4x4 and 5x5x5 cubes this is extended\n"+
		"with lower case letters flubrd which by default indicates\n"+
		"a turn of an inner slice only. For even larger cubes, inner\n"+
		"slices are denoted by subscript notation. If the Multi Slice\n"+
		"box is checked, then the lower case/subscript notation means\n"+
		"a turn of an inner slice and all slices further outwards as\n"+
		"a single unit.\n"+
		"Tip: On a 2x2x2 cube normally all 6 faces can be turned, but\n"+
		"if Multi-Slice is on, only the RFU faces are used.\n"+

		"Printing:\n"+
		"The cube layout might not print correctly on a colour\n"+
		"printer. Make sure that your browser is set up to print\n"+
		"background colours, which is an Internet Options/Advanced\n"+
		"setting in Internet Explorer, or a setting in the Print\n"+
		"dialog in Mozilla Firefox.\n\n"+

		"Written by Jaap Scherphuis, Copyright 2004-2006.");
}

function setForm(){
	document.frm.size.value=size;
	document.frm.len.value=seqlen;
	document.frm.num.value=numcub;
	document.frm.multi.checked=mult;
	document.frm.cubori.checked=cubeorient;
	document.frm.col.value=colorString;

	document.frm.subbutton.focus();
}