格式化金钱，每千分位加逗号

[Rashomon511]

function format(str) {
    let s = ''
    let count = 0
    for (let i = str.length - 1; i >= 0; i--) {
        s = str[i] + s
        count++
        if (count % 3 == 0 && i != 0) {
            s = ',' + s
        }
    }
    return s
}
function format(str) {
    return str.replace(/(\d)(?=(?:\d{3})+$)/g, '$1,')
}