Edit in JSFiddle

function getComplement(x, r){

    if (!x || !r) return [];
    if (typeof x !== 'string') return [];

    x = (x).toString(r);
    var n = x.length;

    return [
        (Math.pow(r, n) - 1 - parseInt(x, r)).toString(r), 
        (Math.pow(r, n) - parseInt(x, r)).toString(r)
    ];
}


console.log(parseInt(getComplement('11', 10)[0], 10));
console.log(parseInt(getComplement('11', 10)[1], 10));

console.log(parseInt(getComplement('00101100', 2)[0], 2));
console.log(parseInt(getComplement('00101100', 2)[1], 2));