Answer 1

var originStr = "红蓝蓝黄红黄蓝红红黄红";
var redCount = originStr.match(/红/ig).length;
var yellowCount = originStr.match(/黄/ig).length;
var blueCount = originStr.match(/蓝/ig).length;
console.log("红".repeat(redCount)+"黄".repeat(yellowCount)+"蓝".repeat(blueCount))

Answer 2

应该是考金额的转换吧，考虑到小数点的情况，如下：
'10000000000.0000'.replace(/d{1,3}(?=(d{3})+(.d*)?$)/g, '$&,')