Implementation of Stack in JavaScript

The Stack data structure is very useful and has many applications. The basic design if that of a linear data structure in which the addition or removal of elements follows the FILO order (first in, last out).

Let’s create a skeleton of a stack class which contains a constructor in which we declare an array to implement our stack.

// Stack class

class Stack {
// We are using an Array to implement our stack
constructor() {
this.items = [];
// Functions to be implemented
// push(item)
// pop()
// peek()
// isEmpty()

We’ve added some methods on our stack class which we would need to implement, allowing us to add items to the stack (push), take an item off our stack (pop), check the last value of the stack (peek), and check whether the stack is empty (isEmpty).

Let’s add these in.

Push(value) – this adds an element to the stack. The push methods has a parameter which is the value of the item we want to add to the stack.

// push function
push(value) {
// push element into the items

Pop() – removes an element from the stack, also returning an error if stack is empty

// pop function
pop() {
// return top most element in the stack
// and removes it from the stack
if(this.items.length == 0) return "Stack is empty";
return this.items.pop();

Peek() – returns the top most element in the stack (doesn’t delete it)

// peek function
peek() {
// return the top most element from the stack
// but doesn't delete it
return this.items[this.items.length - 1];

isEmpty() – return true if the stack is empty

// isEmpty function
isEmpty() {
// return true if stack is empty
return this.items.length == 0;

Let’s run through a quick example of how the we can use our new Stack class.

// Adding element to the stack

// Printing the stack element
// prints [10, 20, 30]

// returns 30

// returns 30 and remove it from stack

// returns [10, 20]


A closer look at the forEach() JavaScript array method

Before the Array.forEach() method, iterating through an array in JavaScript would involve using a for loop.

const beans = ['kidney', 'baked', 'black'];

for (let i = 0; i < beans.length; i++) {

// kidney
// baked
// black

Using Array.forEach() makes things a bit simpler.

beans.forEach(function(bean) {

// kidney
// baked
// black

But what does the Array.forEach() method actually do?

All array methods are functions attached to the Array.prototype object. We want our method to execute a provided function to each element of the array. What would this method look like then? Under-the-hood, the method is still in fact using a for loop.

Array.prototype.forEach = function (callback) {
  if (callback && typeof callback === 'function') {
    for (var i = 0; i < this.length; i++) {
      callback(this[i], i, this);

The for loop will iterate through each item in the array, and pass in the current item in the loop, the current index, and the array itself as arguments to the callback provided. Because the method is attached to the prototype, it can use this to refer to the array itself.

Reimplementing the JavaScript Array.push() method

Adding elements to an array is a very common practice when using JavaScript, made easy by the push() method:

const newArray = ['senna', 'prost'];
// newArray becomes ['senna', 'prost', 'hill']

Let’s try and reimplement a simple version of what the Array.push() method is doing under-the-hood.

In a previous post, we learnt that every JavaScript array has a length property:

newArray.length;  // 3

The Array.length property of an array is the largest integer property name in the array plus one. Remember, the property name refers to the ‘key’ of an element, not the value. The above array is not dissimilar to this object:

const newArray_object = { '0':'senna', '1': 'prost', '2': 'hill'};

We also learnt that we can add an element to the end of array by utilising this length property:

newArray[newArray.length] = 'schumacher';
// newArray becomes ['senna', 'prost', 'hill', 'schumacher']

Given that methods are functions attached to the array prototype object, here is an example of how we could create our our own array object with a push method using our knowledge of the Array.length property:

newArray = {
    length: 0,
    push: function (value) {
        // Place the new value into the
        // next available integer key
        this[this.length] = value;

        // Update the length property
        this.length = this.length + 1;

        // Return the updated length
        return this.length;


newArray[0] //  'woods'
newArray[1] //  'mickleson'
newArray.length  // 2

Arrays in JavaScript

JavaScript’s latest version (ES6) has introduced a number of new Array prototype methods. Over the next few blog posts I’ll go over the most useful of these in detail, including how each of them operate under-the-hood in order to fully understand how they work.

But first off we should understand what an array in JavaScript is. An array in JavaScript is an object. For example, we can create an array using an array literal:

const cricketers = ['amla', 'de villiers', 'smith', 'kohli'];

In our array, the first value will get the property name 0, and the second 1, and so on, allowing us to retrieve value from the array as follows:

cricketers[1];   // 'de villiers'

We can produce a very similar result by creating a new object literal:

const cricketers_object = {
    '0': 'amla',
    '1': 'de villiers',
    '2': 'smith',
    '3': 'kohli',

Both cricketers and cricketers_object are objects, and both have exactly the same property names and values. However, cricketers inherits from the Array.prototype, whereas cricketers_object inherits from the Object.prototype.

JavaScript arrays can contain elements of any value type, including nested arrays and objects:

const assortedArray = ['apple', '0.5', ['nested', 'array'], {'name': 'nested'}, true, false, null, undefined];


The Array.length of an array is the largest integer property name in the array plus one, for example:

assortedArray.length    // 8

The length property can be set explicitly – setting it to less than the current number will delete those with a property name larger than the new length:

assortedArray.length = 2;
// assortedArray becomes ['apple', '0.5']

We can also append items to the end of an array by using:

assortedArray[assortedArray.length] = 'pineapple';
// assortedArray becomes ['apple', '0.5', 'pineapple']

However, we can rather use Array.push method to append items to the end of an array.

Deleting elements

We can use the delete operator on an array to remove elements, but that leaves a hole in the array, as all elements to the right of the deleted item will retain their property names:

delete assortedArray[0];
// assortedArray becomes [ undefined, '0.5', 'pineapple'

When to use an array instead of object?

There is a simple rule for this: if your property names are small sequential integers, use an array. Otherwise, use an object.