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 name='select_time'><option>Select time</option>";
for(var hours=0; hours<24; hours++){
for(var mins=0; mins<60; mins+=30){
let timestamp = hours+":"+mins;
timedropdowndata += "<option>"+timestamp+"</option>";
} // the interval for mins is '30'
} // the interval for hours is '1'
timedropdowndata += "</select>";
I hope it will help you
thank you.