Manual negative indicies of arrays in ES5

es5-negative-indicies.js

/**
 * Manual negative indicies for arrays in ES5:
 * usually we use in such cases only first three
 * negative indices, so these are enough (you may
 * add -4, -5, etc. if needed).
 *
 * See also the implementation with proxies:
 * https://github.com/DmitrySoshnikov/es-laboratory/blob/master/src/array-negative-indices.js
 * 
 * by Dmitry Soshnikov <dmitry.soshnikov@gmail.com>
 * MIT Style License
 */

[-1, -2, -3].forEach(function (negativeIndex) {
    Object.defineProperty(Array.prototype, negativeIndex, {
        get: function () {
            return this[this.length + negativeIndex];
        },
        set: function (value) {
            this[this.length + negativeIndex] = value;
        }
    });
});

var a = [1, 2, 3];
console.log(a[-1]); // 3

a[-1] = 10;
console.log(a); // [1, 2, 10]

"this[this.length - 1]"
=> Shouldn't it rather be something like "this[this.length + negativeIndex]" (maybe a "%this.length". Debatable).

You could extend to any negative number with proxies. My array implementation with proxies: https://github.com/DavidBruant/ProxyArray (beware of potential handler API changes if testing somewhere else than FF4)

Yeah, true, a typo; thanks, fixed.

You could extend to any negative number with proxies

Yes, as I mentioned in the comment to the code, there is my implementation of negative indices based on proxies -- there of course we may handle any index -- https://github.com/DmitrySoshnikov/es-laboratory/blob/master/src/array-negative-indices.js And I made this custom implementation to exclude proxies overhead (usually as is said, we used only first three indices from the end). I saw yours implementation also when you mentioned it on es-discuss, also fits well. About potential changes to the API -- also true, but these are just experiments.

Dmitry.