Printer Errors

Instructions:

In a factory a printer prints labels for boxes. For one kind of boxes the printer has to use colors which, for the sake of simplicity, are named with letters from a to m.
The colors used by the printer are recorded in a control string. For example a "good" control string would be aaabbbbhaijjjm meaning that the printer used three times color a, four times color b, one time color h then one time color a...
Sometimes there are problems: lack of colors, technical malfunction and a "bad" control string is produced e.g. aaaxbbbbyyhwawiwjjjwwm with letters not from a to m.
You have to write a function printer_error which given a string will return the error rate of the printer as a string representing a rational whose numerator is the number of errors and the denominator the length of the control string. Don't reduce this fraction to a simpler expression.

The string has a length greater or equal to one and contains only letters from a to z.

Examples:
s="aaabbbbhaijjjm"
printer_error(s) => "0/14"

s="aaaxbbbbyyhwawiwjjjwwm"
printer_error(s) => "8/22"

Thoughts:

1.I split the string into an array with each letter being an element and filter all the letters between n and z into a new errors array.
2.I return the recurrence of errors compared with the length of the initial string.

Solution:

function printerError(s) {
  const errors = s.split('').filter(letter =>letter.match(/[n-z]/g))
    return `${errors.length}/${s.length}`;
}

This is a CodeWars Challenge of 7kyu Rank