Adding our own custom methods in JavaScript prototype class

I have a question regarding JavaScript.

is this good practice to add our own method in prototype class

like I have added my own custom method in String prototype class like this

String.prototype.mirror = function () {
  let str = this.toString() ;
 return str.split('').reverse().join('')
}

and now i am using my own method with any string value like

"hello".mirror()
"olleh"