SOURCE

var num1=12345.33
//有小数的 。小数点后面的不用分隔
function fomat(num){
    let str=num.toString()
    let decimals=''
    if(str.indexOf('.')>-1){
        let arr=str.split('.')
        str=arr[0]
        decimals=arr[1]
        console.log(decimals)
    }
    let length=str.length
    if(length<=3){
        return str
    }
    let temp=decimals?'.'+decimals:''
    let remainder=length%3
    if(remainder>0){
        return str.slice(0,remainder)+','+str.slice(remainder,length).match(/\d{3}/g).join(',')+temp
    }else{
        return str.slice(0,length).match(/\d{3}/g).join(',')+temp
    }
}
let num2=fomat(num1)
console.log(num2)

var num3=12345678
//没有小数的
function fomat2(num){
    let str=num.toString()
    let length=str.length
    let remainder=length%3
    if(length<=3){
        return str
    }
    if(remainder>0){
        return str.slice(0,remainder)+','+str.slice(remainder,length).match(/\d{3}/g).join(',')
    }else{
        return str.slice(0,length).match(/\d{3}/g).join(',')
    }
}
let num4=fomat(num3)
console.log(num4)