function formatMoney(num){
    return (+num).toLocaleString("en-US");
}

const log = console.log;

log(formatMoney(123456789));  // 123,456,789
log(formatMoney(6781)) // 6,781
log(formatMoney(5)) // 5
log(formatMoney(19999999933333333333333)) // 19,999,999,933,333,333,000,000

console 命令行工具 X clear

console