Below you can check the example of concate data in variable through for loop in jquery
Please refer '+=' in the code for concatenation example

var timedropdowndata = "Select time";
for(var hours=0; hours<24; hours++){
  for(var mins=0; mins<60; mins+=30){
    let timestamp = hours+":"+mins;
    timedropdowndata += ""+timestamp+"";

  } // the interval for mins is '30'
} // the interval for hours is '1'
timedropdowndata += "";

I hope it will help you
thank you.