[devote]

ну если так нужны функции то вот:

function toBIN( num ) {
    var out = "", bit = 1;
    while( num >= bit ) {
        out = ( num & bit ? 1 : 0 ) + out;
        bit <<= 1;
    }
    return out || "0";
}

alert( toBIN(20) );

function toDEC( dec ) {
    var out = 0, len = dec.length, bit = 1;
    while( len-- ) {
        out += dec[ len ] == "1" ? bit : 0;
        bit <<= 1;
    }
    return out;
}

alert( toDEC( '10100' ) );