ES6 List negative indices

es6-list-negative-indices.js

/**
 * Reverse indices of Lists/Arrays in ES6.
 *
 * by Dmitry Soshnikov <[email protected]>
 * MIT Style License
 */

'use strict';

class List extends Array {
  peek() {
    return this._reverseGet(1);
  }

  get '-1'() {
    return this.peek();
  }

  get '-2'() {
    return this._reverseGet(2);
  }

  get '-3'() {
    return this._reverseGet(3);
  }

  set '-1'(v) {
    return this._reverseSet(1, v);
  }

  set '-2'(v) {
    return this._reverseSet(2, v);
  }

  set '-3'(v) {
    return this._reverseSet(3, v);
  }

  _reverseGet(n) {
    return this[this.length - n];
  }

  _reverseSet(n, v) {
    return this[this.length - n] = v;
  }
}

// -----------------------------------
// __tests__

let list = new List(1, 2, 3);

console.log(
  list[-1], // 3
  list[-2], // 2
  list[-3], // 1

  list.peek() // 3
);

list[-1] = 5;

console.log(list); // [1, 2, 5]

console.log(
  list instanceof Array, // true
  Array.isArray(list)    // true
);