const buffer = Buffer.from("9999999999", "ascii");

const mask = 15; // 00001111
let result = 0;

for (let i = 0; i < buffer.length; i++) {
    const place = 1 * (10 ** i);
    const pos = (buffer.length - i) - 1;

    result += (buffer[pos] & mask) * place;
}

console.log(result);