After experiment, we find out that array access is even a little more efficient than reference access.

js-array-dictionary-compare.js

'use strict';
var dict = [];
var dict2 = {};
var i, j, t0, t1;

var ICNT = 1000;
var JCNT = 1000;

t0 = performance.now();
for (i = 0; i <= ICNT; i += 2) {
    dict[i] = [];
    for (j = 0; j <= JCNT; j += 2) {
        dict[i][j] = Math.random();
    }
}
for (i = ICNT; i > 0; i -= 8) {
    for (j = ICNT; j > 0; j -= 8) {
        dict[i][j] = Math.random();
    }
}
t1 = performance.now();
console.log("Write to 2D array " + (t1 - t0) + " ms.");

t0 = performance.now();
for (i = 0; i <= ICNT; i += 2) {
    for (var j = 0; j <= JCNT; j += 2) {
        dict2[i + ',' + j] = Math.random();
    }
}
for (i = ICNT; i > 0; i -= 8) {
    for (j = JCNT; j > 0; j -= 8) {
        dict2[i + ',' + j] = Math.random();
    }
}
t1 = performance.now();
console.log("Write to sringMap " + (t1 - t0) + " ms.");

var ICNT = 100000;
var i;
var t0, t1;

function Data(nx) {
    this.next = nx;
    this.str = "hhh";
    this.num = 3;
}


var array = [];
var refs = [];
var randomIndex = [];
for (i = 0; i < ICNT; i++) {
    randomIndex.push(Math.floor(Math.random() * ICNT));
}

for (i = 0; i < ICNT; i++) {
    array.push(new Data(randomIndex[i]));
}

for (i = 0; i < ICNT; i++) {
    refs.push(new Data());
}

for (i = 0; i < ICNT; i++) {
    refs[i].next = refs[randomIndex[i]];
}

function roughSizeOfObject(object) {
    var objectList = [];
    var stack = [object];
    var bytes = 0;

    while (stack.length) {
        var value = stack.pop();

        if (typeof value === 'boolean') {
            bytes += 4;
        }
        else if (typeof value === 'string') {
            bytes += value.length * 2;
        }
        else if (typeof value === 'number') {
            bytes += 8;
        }
        else if
        (
            typeof value === 'object'
            && objectList.indexOf(value) === -1
        ) {
            objectList.push(value);

            for (var i in value) {
                stack.push(value[i]);
            }
        }
    }
    return bytes;
}

function itArray() {
    var sum = 0;
    for (var i = 0; i < array.length; i++) {
        sum += array[array[i].next].num;
    }
    return sum;
}

function itRefs() {
    var sum = 0;
    for (var i = 0; i < refs.length; i++) {
        sum += refs[i].next.num;
    }
    return sum;
}

t0 = performance.now();
console.log(itArray());
t1 = performance.now();
console.log("Call to itArray " + (t1 - t0) + " milliseconds.");
console.log("size:" + roughSizeOfObject(array));

t0 = performance.now();
console.log(itRefs());
t1 = performance.now();
console.log("Call to itRefs " + (t1 - t0) + " milliseconds.");
console.log("size:" + roughSizeOfObject(refs));