千分位分隔符

const thousandth = (num = 123456789, fixed = 0) => {
    const strNum = num.toFixed(fixed)
    const [startStr, endStr] = strNum.split('.')
    let endIdx = startStr.length - 1
    let n = 0
    const res = []
    while(endIdx > -1){
        res.unshift(startStr[endIdx])
        n++
        if(n % 3 === 0 && endIdx !== 0){
            res.unshift(',')
        }
        endIdx--
    }
    return endStr ? `${res.join('')}.${endStr}` : res.join('')
}

　　从后往前 三位一切

const thousandth = (num = 123456789, fixed = 2) => {
    if (typeof num !== 'number') return new Error('请传入数字')
    const [startStr, endStr] = num.toFixed(fixed).split('.')
    const res = []
    for (let i = startStr.length; i > -1; i -= 3) {
        if (i > 3) {
            res.unshift(',', ...startStr.slice(i - 3, i))
        } else {
            res.unshift(...startStr.slice(0, i))
        }
    }
    return endStr ? `${res.join('')}.${endStr}` : res.join('')
}