Truncate a string - Javascript

truncate.js

truncate (str, length, ending) {
    if (length == null) {
      length = 100;
    }
    if (ending == null) {
      ending = '...';
    }
    if (str.length > length) {
      return str.substring(0, length - ending.length) + ending;
    } else {
      return str;
    }
};
console.log(truncate('JS string exercises.'))
console.log(truncate('JS string exercises.',19))
console.log(truncate('S string exercises.',15,'...'))