Apex - solution to add leading zeros

 function pad(

  x, // input number to pad

  y // input number for trunc

){

  return (

    1e16 + x + "" // concatenate a big number to the input number and converting to string

  ).slice(-y) // trunc last Y numbers

}

Output

pad(1,2) => 01

pad(123,2) => 23

pad(123,5) => 00123