function replaceAll(str , replaceKey , replaceVal){
    var reg = new RegExp(replaceKey , ‘g‘);//g就是代表全部
    return str.replace(reg , replaceVal || ‘‘);
}

//test
var str1 = replaceAll(‘abcd,abc,aa,abc‘ , ‘a‘);//去除所有a

//test2
var str2 = replaceAll(‘abcd,abc,aa,abc‘ , ‘a‘ , ‘e‘);//所有的a替换成e