readme.md

Stack Implementation

Now that we know fundamentally how a stack works, we're going to implement one in JavaScript. We will create a Stack Class that has methods such as push, pop and peek.

Stack Class & Constructor

Let's start by creating a class called Stack. In the constructor, we'll create a property called maxSize and set it to 100. This is the maximum number of elements that can be in the stack. We'll also create a property called stack and set it to an empty array. This is where we'll store the elements in the stack. Finally, we'll create a property called top and set it to -1. This is the index of the top element in the stack. We are using -1 because the stack is empty. The first element will be at index 0.

class Stack {
  constructor() {
    this.maxSize = 100;
    this.stack = [];
    this.top = -1;
  }
}

push Method

Create the push method to add to the stack. It will take in a value (element). First, check if the stack is full. If it is, return false. Increment the top index by 1. Set the element at the top index of the stack to the value. Return true.

 push(value) {
    if (this.isFull()) {
      return false;
    }
    this.top++;
    this.stack[this.top] = value;

    return true;
  }

isFull Method

Of course we need to add the method to check if the stack is full. It will return true if the top index is equal to the maximum size minus 1. Otherwise, it will return false.

isFull() {
    return this.top === this.maxSize - 1;
  }

Let's also add a method to check if the stack is empty. It will return true if the top index is equal to -1. Otherwise, it will return false.

isEmpty() {
    return this.top === -1;
  }

Now we can test it out a little bit. In the run file, create a new instance of the Stack class. Push a few elements to the stack. Then, log the stack to the console.

const stack = new Stack();

stack.push(1);
stack.push(2);
stack.push(3);

console.log(stack);

You should see something like this:

Stack {
  maxSize: 100,
  stack: [ 1, 2, 3 ],
  top: 2
}

pop Method

Now let's create the pop method. It will remove the top element from the stack. It is important to know that pop and just about any method of any structure can be implemented in many ways. Sometimes the actual data will not be removed from the array (this.stack). It will just go by what is in the top of the stack. I want the actual data to be removed, so I will use the built in pop method on the array.

First, check if the stack is empty. If it is, return null. Then decrement the top by 1 and then return the result from this.stack.pop().

pop() {
    if (this.isEmpty()) {
      return null;
    }
    this.top--;

    return this.stack.pop();
  }

peek Method

Finally, let's create the peek method. It will return the top element of the stack. First, check if the stack is empty. If it is, return null. Return the element at the top index of the stack.

peek() {
    if (this.isEmpty()) {
      return null;
    }
    return this.stack[this.top];
  }

Now let's test some more.

const stack = new Stack();

stack.push(1);
stack.push(2);
stack.push(3);

console.log(stack);

console.log(stack.pop());
console.peek();
console.log(stack.pop());
console.peek();
console.log(stack.pop());