NHS Number Validator in JavaScript

isValidNHSNumber.js

function isValidNHSNumber(number) {

  number = (number + '').replace(/\D/g, '');
  
  if(number.length == 10) {
    
    let total = 0;

    // STEP 1: Starting from the left, multiply each of the first nine digits by (11 - digit position).
    for(let i = 1; i <= 9; i++) {
      const digit = number.substr(i - 1, 1);
      const factor = 11 - i;
      // STEP 2: Add the results of each multiplication together.
      total += (digit * factor);
    }

    // STEP 3: Divide the total by 11 and establish the remainder.
    const reminder = total % 11;

    // STEP 4: Subtract the remainder from 11 to give the check digit. If the result is 11 then a check digit of 0 is used. If the result is 10 then the NHS number is invalid and not used.
    let checkDigit = 11 - reminder;

    if(checkDigit == 11) { 
      checkDigit = 0; 
    }

    // STEP 5: Check if the check digit matches the last digit. If it does not, the NHS number is invalid.
    if(checkDigit == number.substr(9, 1)) { 
      return true;
    }

  }
  
  return false;
}