Check Digit JavaScript function for modulo 11

checkDigit11.js

/*
  Check digit using modulo 11
  Arguments:
    - n is your given number to test the check digit
    - x is an optional argument that is used to determine the check digit
      used in case of the rest of the division is less than 2. Defaults to 0.
      
  Returns an integer, the checkDigit for the given number.
*/
var checkDigit11 = function(n, x) {

  var l = n.length, i = 0, j = (l%8), v = 0;

  for(i = 0, l = l-1; i < l; i++) {
    v += parseInt(n[i], 10) * j; 
    j = (j == 2) ? 9 : --j;
  }

  return v = (v%11 < 2) ? (x || 0) : (11 - (v%11));
};

K???

Why are you returning the value of an assignment ? v = (v%11 < 2) ? ... this is bad practice and makes the code uselessly confusing here

Function is also not working correctly because j goes negative for strings that are 8 chars or more, the function also doesn't actually return the check of the last digit, it only returns the expected one