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In some programming languages, we can access array elements using negative indexes, counted from the end.

Like this:

          let array = [1, 2, 3];

array[-1]; // 3, the last element
array[-2]; // 2, one step from the end
array[-3]; // 1, two steps from the end
        

In other words, array[-N] is the same as array[array.length - N].

Create a proxy to implement that behavior.

Thats how it should work:

          let array = [1, 2, 3];

array = new Proxy(array, {
  /* your code */
});

alert( array[-1] ); // 3
alert( array[-2] ); // 2

// Other array functionality should be kept "as is"
        

solution

let array = [1, 2, 3];

array = new Proxy(array, {
  get(target, prop, receiver) {
    if (prop < 0) {
      // even if we access it like arr[1]
      // prop is a string, so need to convert it to number
      prop = +prop + target.length;
    }
    return Reflect.get(target, prop, receiver);
  }
});


alert(array[-1]); // 3
alert(array[-2]); // 2

Accessing array[-1]