Get the element of an array that has an index given by a variable in JavaScript

function Oo(s) {
    this._s = s;
}

Oo.prototype.toString = function() {
    return "-" + this._s+"-";
}

var a = [];
a.push(new Oo('x'));
a.push(new Oo('y'));
a.push(new Oo('z'));

a[3] = a[1];
alert(a);  // -x-,-y-,-z-,-y-

a[3]._s = 'XX'; // let's change the newly added item
alert(a); // -x-,-XX-,-z-,-XX- <- 3 and 1 points to the same object!

a.splice(1, 1); // remove one element, starting from index = 1
alert(a); // -x-,-z-,-XX-

var arrayName = <?php echo json_encode($arrayName); ?>;
$(".elementClass").each(function(index, id) {
    $(id).html(arrayName[index-1]);
});

function indexOfPartial(array, value) {
    for (var i = 0; i < array.length; i++) {
        if (array[i].indexOf(value) >= 0) {
            return i;
        }
    }
    return -1;
}

var index = indexOfPartial(mesreponses, 'NB - Nude');

function count(myguid, index) {
    const query = {"guid": myguid};
    query["v." + index + ".cnt"] = {$exists: true};

    return db.MuCollection.count(query);
}

count("some-guid", 3);

const arr1 = [0,1,1,0,1];
const arr2 = ["a","b","c","d","e"];

let res = arr1.reduce(
    (acc, v, idx) => v ? acc.concat({val: arr2[idx], idx}) : acc,
    []
);

console.log(res);