A stack in JavaScript with ES2015

stack.js

// Note: Save as stack.js and run 'babel-node stack.js' to display in the command window. This is written in ES2015
((global) => {

    // Note: I'm aware .pop() and .push() exist natively with JavaScript arrays, but where's the fun in that? =)

    /**
     * Stack interface
     */
    function Stack() {
        this._stack = [];
        this._index = -1;
    }

    /**
     * Append functions to the Stack prototype chain
     * @type {object}
     */
    Stack.prototype = {
        /**
         * Peek at the last item placed last on the stack
         *
         * @return {mixed|null} Last item placed on the stack; otherwise, null
         */
        peek: () => {
            const index = this._index;
            if (index === -1) {
                return null;
            }

            return this._stack[index];
        },

        /**
         * Pop the last item off the stack
         *
         * @return {mixed|null} Last item placed on the stack; otherwise, null
         */
        pop: () => {
            if (this._index === -1) {
                return null;
            }

            // Return the data at the current index and decrement the index too
            return this._stack[this._index--];
        },

        /**
         * Puah data on to the stack
         *
         * @param {mixed} data Data to push on to the stack
         * @return {undefined}
         */
        push: (data) => {
            // Increment the index and push the data on to the stack at the index value
            this._stack[++this._index] = data;
        },

        // Helper functions

        /**
         * Get the number of items on the stack
         *
         * @return {number} Number of items on the stack
         */
        count: () => {
            return this._index + 1;
        },
    };

    // Example

    (() => {
        // Create a stack
        const stack = new Stack();
        global.console.log('Stack peek: ', stack.peek());
        global.console.log('Stack pop: ', stack.pop());
        global.console.log('Stack count: ', stack.count());

        // Push primitive datatypes e.g. string, number
        stack.push('Some data 1');
        stack.push('Some data 2');
        stack.push('Some data 3');

        global.console.log('Stack peek: ', stack.peek());

        global.console.log('Stack count: ', stack.count());
        global.console.log('Stack pop: ', stack.pop());
        global.console.log('Stack pop: ', stack.pop());
        global.console.log('Stack pop: ', stack.pop());

        // There should be no items left on the stack
        global.console.log('Stack pop: ', stack.pop());
        global.console.log('Stack count: ', stack.count());

        // Push on to the stack again
        stack.push('Some data 1');
        global.console.log('Stack peek: ', stack.peek());
        global.console.log('Stack count: ', stack.count());
    })();

})(window); // An IIFE for function scope i.e. creating a namespace