valueOf.js

//custom string manipulation class
class MyString {
  constructor(str) {
    this.str = str;
  }
  replaceVowels(replacement) {
    this.str = this.str.replace(/a|e|i|o|u/gi, replacement);
  }
  //The valueOf() method returns the primitive value of the specified object.
  valueOf() {
    return this.str;
  }
}

//instantiate it
let myStr = new MyString('test');

//call custom method
myStr.replaceVowels('*');

//now interact with it like it's a normal string
let newStr = 'hi there ' + myStr;
//=> "hi there t*st"