// Does not handle negative numbers.
function intToBin(number) {
    if (number === 0) {
        return '0';
    }
    let res = '';
    while (number > 0) {
        res = String(number % 2) + res;
        number = parseInt(number / 2, 10);
    }
    return res;
}

console.log(intToBin(0) === 0..toString(2) && 0..toString(2) === '0');
console.log(intToBin(1) === 1..toString(2) && 1..toString(2) === '1');
console.log(intToBin(2) === 2..toString(2) && 2..toString(2) === '10');
console.log(intToBin(3) === 3..toString(2) && 3..toString(2) === '11');
console.log(intToBin(5) === 5..toString(2) && 5..toString(2) === '101');
console.log(intToBin(99) === 99..toString(2) && 99..toString(2) === '1100011');