let h = +prompt("Enter current hour: ");
let min = +prompt("Enter current minutes: ");
let truk = +prompt("Enter duration (minutes): ");

let min1 = truk % 60; // We get minutes from duration
let h1 = Math.trunc(truk / 60); // We get hours from duration

console.log("You have entered: " + "h = " + h + ", " + "min = " + min + ", " + "truk = " + truk);
//without if
var result1 = new Date(2021, 0, 1, h, min, 00, 000); //set time according to user inputs
result1.setMinutes(result1.getMinutes() + truk); //add minutes to set time
console.log("Result: " + (result1.getHours()) + " : " + (result1.getMinutes()));

you need to check if minutes are going over 60. If yes then add an hour to existing hrs. Same logic must be applied if hrs are going beyond 24. Try this :

let h = +prompt("Enter current hour: ");
let min = +prompt("Enter current minutes: ");
let truk = +prompt("Enter duration (minutes): ");

let min1 = truk % 60; // We get minutes from duration
let h1 = Math.trunc(truk / 60); // We get hours from duration

console.log("You have entered: " + "h = " + h + ", " + "min = " + min + ", " + "truk = " + truk);
let min_final = 0
let hr_final = h
if((min + min1) > 60){
  min_final = (min + min1) - 60
  hr_final = hr_final + 1
}
hr_final = hr_final + h1
if(hr_final >= 24){
  hr_final = 24 - hr_final
}
if(hr_final < 10){
  hr_final = "0" + hr_final
}
console.log("Result: " + (hr_final) + " : " + (min_final));

You're on the right track, but you need to consider some additional factors:

1. You need to use the remainder operator % again to ensure that the values you return back are base-24 (for hours) and base-60 (for minutes).
2. You also need to consider the updated "hours" value in this scenario is not just the current hour plus the number of hours in the duration you provide, but also if the duration causes the minutes to overflow past 60 as well (for instance, a duration of 10 at 17:55 should return 18:05, not 17:05).
3. Finally, most locales express minutes with a leading zero when the value is less than 10 - a bit of string manipulation with String.substr() can go a long way in this scenario.

let h = +prompt("Enter current hour: ");
let min = +prompt("Enter current minutes: ");
let truk = +prompt("Enter duration (minutes): ");

let newMin = ('0' + (min + (truk % 60)) % 60).substr(-2);
let newH = (h + Math.trunc((truk + min) / 60) + Math.trunc(truk / 60)) % 24;

console.log("You have entered: " + "h = " + h + ", " + "min = " + min + ", " + "truk = " + truk);
console.log("Result: " + newH + " : " + newMin);