How to add 0 in timer when less than 10?

Question

JavaScript

It is necessary that when a digit enters less than 10 digit, then the digit would not be 1, but 01 should be. My implementation
https://jsfiddle.net/hollanditkzn/0qvwke7o/56/

window.onload = function(){ 
  function timer(){
    let minute = document.getElementById('minute').innerHTML;
    let second = document.getElementById('second').innerHTML;
    let end = false;
    
    if( second > 0 ) {
    second--;
    if( second < 10){
      '0'+second;
    }
    } else{
      second = 60;
      
      if( minute > 0 ) {
      minute--;
      if(minute < 10){
      	'0'+minute
      }
      } else{
        end = true;
      }
    }

    if(end){
      clearInterval(intervalID);
      alert("Время истекло!");
    }else{document.getElementById('minute').innerHTML = minute;
      document.getElementById('second').innerHTML = second;
    }
  }
  window.intervalID = setInterval(timer, 1000);
}

<div id='timer'>
  <div id='minute'>01</div>:
  <div id='second'>10</div>
</div>

Answer 1

It's easier to make a separate function:

function pad(x) {
  return x < 10 ? '0' + x : x;
}

and use in your code like this:

document.getElementById('minute').innerHTML = pad(minute);
document.getElementById('second').innerHTML = pad(second);

Or you can use the standard one (but not supported by older browsers, if you need their support, do not forget to add the polyfill from the above article).