JSFiddle - React, Tailwind, and code Playground

JavaScript

var time = {
  "timetable": {
    "MONDAY": {
      "start-end0": {},
      "start-end1": {},
      "start-end2": {},
      "start-end3": {},
      "start-end4": {}
    }
  }
}

for(var i=0;i<5;i++)
  time.timetable.MONDAY["start-end"+i] = {}
  console.log(time)