JavaScript: Custom iterator

// e.g., inject something whenever iterating a list
const arr = [ 1, 2, 3 ];
arr[Symbol.iterator] = function () {
  let i = 0;
  let arr = this;
  return {
    next: function () {
      if (i >= arr.length) {
        return { done: true };
      } else {
        const value = arr[i] + "🎵";
        i++;
        return { value, done: false };
      }
    }
  };
};
for (const x of arr) { console.log(x); } // => 1🎵\n2🎵\n3🎵