function intToHex(num){
    switch(num){
        case 10: return "A";
        case 11: return "B";
        case 12: return "C";
        case 13: return "D";
        case 14: return "E";
        case 15: return "F";
    }
    return num;
}

function decimalToHex(num){
    let hex_out = [];
    while(num > 15) {
        hex_out.push(intToHex(num%16));
        num = Math.floor(num / 16);
    }
    return intToHex(num) + return hex_out.join("");
}

// test cases
console.log(decimalToHex(999098) === "F3EBA");
console.log(decimalToHex(123) === "7B");