How to convert a number to time?

Question

I'm trying to create a function that takes a number and returns a timestamp (HH:mm) using date-fns version 1.30.1 or plain JavaScript.

What I'm trying to achieve is to help a user when entering a time. I'm using Vue.js to update the field when a user moves away from the input field. So if a user types 21 then moves away, the field would ideally update to 21:00.

Some examples would be:

21 = 21:00  
1 = 01:00  
24 = 00:00  
2115 = 21:15  

Numbers like 815 does not have to return 08:15. Numbers like 7889 should return an error.

I have tried using regex:

time = time
    .replace(/^([1-9])$/, '0$1')
    .replace(/^([0-9]{2})([0-9]+)$/, '$1:$2')
    .replace(/^24/, '00:00')

I have also tried using the parse method in date-fns but can't seem to wrap my head around how to solve this.

Answer 1

Conversion based on <100 (hours-only) and >=100 (100*hours+minutes), plus some fight with 24 and single-digit numbers (both hours and minutes):

function num2time(num){
  var h,m="00";
  if(num<100)
    h=num;
  else {
    h=Math.floor(num/100);
    m=("0"+(num%100)).slice(-2);
  }
  h=h%24;
  return ("0"+h).slice(-2)+":"+m;
}

console.log(num2time(8));
console.log(num2time(801));
console.log(num2time(24));
console.log(num2time(2401));
console.log(num2time(2115));
console.log(num2time("8"));
console.log(num2time("2115"));

---

Original answer, kept for the comment only, but would not handle 24 or single-digit minutes correctly:

For example you can do a very mechanical conversion

function num2time(num){
  if(num<10)
    t="0"+num+":00";
  else if(num<100)
    t=num+":00";
  else {
    if(num<1000)
      t="0"+Math.floor(num/100);
    else if(num<2400)
      t=Math.floor(num/100)
    else
      t="00";
    t+=":"+(num%100);
  }
  return t;
}

console.log(num2time(8));
console.log(num2time(2115));
console.log(num2time("8"));
console.log(num2time("2115"));

---

Example verification:

function num2time(num){
  var h,m="00";
  if(num<100)
    h=num;
  else {
    h=Math.floor(num/100);
    m=("0"+(num%100)).slice(-2);
  }
  if(h<0 || h>24) throw "Hour must be between 0 and 24"
  if(m<0 || m>59) throw "Minute must be between 0 and 59"
  h=h%24;
  return ("0"+h).slice(-2)+":"+m;
}

var numstr=prompt("Enter time code");
while(true) {
  try {
    console.log(num2time(numstr));
    break;
  } catch(ex) {
    numstr=prompt("Enter time code, "+numstr+" is not valid\n"+ex);
  }
}