Implementation of a Stack in JS

Stack.js

/**
 * Implementation of a Stack in Javascript
 * @constructor
 * @author Chandra Mohan
 */
function Stack() {
    /**
     * Element to store data. _ is used to imply that clients should not modify it.
     * @type {Array}
     * @private
     */
    this._storeArray = [];

    /**
     * Index of the array denoting top position. _ is used to imply that clients should not modify it.
     * @type {number}
     * @private
     */
    this._index = -1;
}

// Now add Prototype methods.
Stack.prototype = {
    /**
     * Utility method to return _index. Note _index is denoted to be private. But so are these methods.
     * @returns {number}
     * @private
     */
    _getIndex: function () {
        return this._index;
    },

    _setIndex : function (indexValue) {
        this._index = indexValue;
    },

    _getData : function () {
        return this._storeArray[this._index];
    },

    _addData : function (value) {
        this._storeArray[this._index] = value
    },

    /**
     * Push an element into Stack
     * @param val
     */
    push : function (val) {
        this._setIndex(this._getIndex() + 1);
        this._addData(val);
        console.log("Pushed: " + val);
    },

    /**
     * Pop an element out of Stack
     * @returns {null}
     */
    pop : function () {
        if (this._getIndex() < 0) {
            return null;
        } else {
            console.log("Popping: " + this._getData());
            this._setIndex(this._getIndex() - 1);
        }
    },

    /**
     * Check if the stack is empty.
     * @returns {boolean}
     */
    isEmpty : function () {
        console.log("isEmpty === " + (this._getIndex() < 0) + "   " + "Current Index=" + this._getIndex());
        return this._getIndex() < 0;
    }
}

// TEST CODE
var myStack = new Stack();
console.log("myStack: " + myStack);
console.log("myStack.index: " + myStack._index);
myStack.push(201);
myStack.push(202);
myStack.push(203);
myStack.push(204);
myStack.pop();
myStack.pop();
myStack.isEmpty();
myStack.pop();
myStack.pop();
myStack.isEmpty();

var myStack1 = new Stack();
console.log("myStack1: " + myStack1);
myStack1.push(301);
myStack1.push(302);
myStack1.pop();
myStack1.push(303);
myStack1.push(304);
myStack1.pop();
myStack1.pop();
myStack1.isEmpty();
myStack1.pop();
myStack1.isEmpty();