Previous multiple of three.js

// DESCRIPTION:
// Given a positive integer n: 0 < n < 1e6, remove the last digit until you're left with a number that is a multiple of three.

// Return n if the input is already a multiple of three, and if no such number exists, return null, a similar empty value, or -1.

// Examples
// 1      => null
// 25     => null
// 36     => 36
// 1244   => 12
// 952406 => 9

//  -----MySolution-----
const prevMultOfThree = n => {
    while (n >= 1){
      if (n % 3 === 0){
        return n
      } else if(Math.floor(n/10) >= 1){
        n = Math.floor(n/10)
        } else {
          return null
      }
    }
  }