myNumber.toString( [ radix ] ) in ECMAScript 262

If radix is undefined, base 10 is assumed. The following code shows this method being used to express numbers in various bases:

var numbers = [8,10,12,16,200,255];
var bases = [2,8,10,16];
var msg="";
for (var i=0,len=numbers.length;i<len;i++){
    var n = numbers[i];
    for (var j=0,len2=bases.length;j<len2;j++){
        var base = bases[j];
        msg += n+' in base '+base+' is '+n.toString(base)+'\n';
    }
}
/****************************************
msg is:
8 in base 2 is 1000
8 in base 8 is 10
8 in base 10 is 8
8 in base 16 is 8
10 in base 2 is 1010
10 in base 8 is 12
10 in base 10 is 10
10 in base 16 is a
12 in base 2 is 1100
12 in base 8 is 14
12 in base 10 is 12
12 in base 16 is c
16 in base 2 is 10000
16 in base 8 is 20
16 in base 10 is 16
16 in base 16 is 10
200 in base 2 is 11001000
200 in base 8 is 310
200 in base 10 is 200
200 in base 16 is c8
255 in base 2 is 11111111
255 in base 8 is 377
255 in base 10 is 255
255 in base 16 is ff
****************************************/

Note that the returned string is not padded to any number of digits. The following convenience method shows how to extend the Number prototype to support a new method, which allows you to specify a certain number of digits desired; if less than that number is returned by the call to toString(), 0s are padded to the front.

Number.prototype.convertTo=function(base,padTo){
    var s=this.toString(base);
    if (!padTo || s.length>=padTo) return s;
    return Math.pow(10,padTo-s.length).toString().slice(1)+s;
}
var x = 12;
x.toString(2);     // ** "1100"
x.convertTo(2,8);  // ** "00001100"
x.convertTo(16,2); // ** "0c"