var num = '12334534534.43234'
    console.log(formatNum(num))

    function formatNum(num){
        num += ''
        // 如果包含,.或者小于1000
        if(num.indexOf(',') > 0 || num*1<1000) return num
        // 拆分字符
        var arr = num.split('.')
        var startStr = arr[0]
        var endStr = arr[1] ? '.'+arr[1] : ''
        var arr = startStr.split('').reverse()
        var s = ''
        for(var i=0;i<arr.length;i++){
            if(i!=0 && i%3==0){
                s+=','
            }
            s+=arr[i]
        }
        num = s.split(',').reverse().toString() +endStr
        return num
    }