js 里的replace与replaceall

js replace函数 的用法：

var a = "MOPQ";
var b = a.replace("M","m");

注：将a字符串里的"M",替换为"m"

js replaceAll:
js 未提供replaceAll 函数，但可以用以下方法实现

var a="12,ss，sdfjsd，sdsdfj,sfjs";
var b = a.replace(new RegExp("，","gm"),",");

注:将a字符串里的"，"全部替换为","