The Array.prototype property allows new properties and methods to be added to existing arrays.

<!-- html element to place output -->
<p id="my_cars"></p>

// array
let cars = ['Lamborghini', 'Ferrari', 'Maserati', 'Alfa Romeo'];

// add new method
Array.prototype.my_upper_case = function() {
  for (let i = 0; i < this.length; i++) {
    this[i] = this[i].toUpperCase();
  }
};

// use new method on array
cars.my_upper_case();

//output to HTML element
document.getElementById('my_cars').innerHTML = cars;

LAMBORGHINI,FERRARI,MASERATI,ALFA ROMEO

Array.prototype.name = value