Object properties Inheritance using prescribed method

object-extending.js

function DefaultStrFns(someStr) {
     let arr;
     this.splitIntoArray = function() {
          arr =  someStr.split('');
          return arr;
     }
  
     this.concatFromArray = function() {
          return arr.join('');
     }
}

function ExtendStrFns(someStr) {
     defaultStrFns.call(this, someStr);
     this.splitIntoArrayUsingComa = function() {
          return someStr.split(',');
     }
}

function extendedStringFns(someStr) {
    return new ExtendStrFns(someStr);
}