方法一（被处理的必须为 Number

let str = 123456789;
y.toLocaleString() //123,456,789

方法二（被处理的为String类型的数字）

let reg = /(\d)(?=(?:\d{4})+$)/g;

let str = '123456789';

let target = str.replace(reg, '$1,')  //1,2345,6789