first solution you split the string to obtain the two values in an Array :
var str = "required:true,validType:'timegt['#timeofdaymeterslotonebegintime,#timeofdaymeterslotoneendtime']";
var arr = str.split(",");
var result = [];
result.push(arr[0]);
result.push(arr.filter((element, index) => (index>0)).join());
console.log(result);
second solution you extract from the initial string two strings containing your values :
var str = "required:true,validType:'timegt['#timeofdaymeterslotonebegintime,#timeofdaymeterslotoneendtime']";
var index = str.indexOf(",");
var result1 = str.slice(0, index);
var result2 = str.slice(index+1);
console.log(result1);
console.log(result2);