JavaScript Data Structures: Stack: Push / Add a new node
Tags: beginners, tutorial, javascript, webdev
Published: 2019-12-19 07:37:06

---
title: "JavaScript Data Structures: Stack: Push / Add a new node"
description: "JavaScript Data Structures: Stack: Push / Add a new node"
published: true
tags: ["Beginners", "Tutorial", "JavaScript", "Webdev"]
series: JavaScript Data Structures
---

## Intro

[Last time](https://dev.to/miku86/javascript-data-structures-stack-intro-51la), we learned what a Stack is and set it up.

Today, we learn how to push / add a new node on top of the Stack.

---

## Starter Code

We start with the code [from the last part](https://dev.to/miku86/javascript-data-structures-stack-intro-51la).

```js
class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class Stack {
  constructor() {
    this.length = 0;
    this.last = null;
  }
}
```

---

## Thoughts

First, we should think about the constraints and possibilities:

If the Stack is empty:

- create a new node
- set the new node as the last node
- increase the stack's length by 1
- return the new node

All remaining cases:

- create a new node
- set the current last node as the new node's next node
- set the new node as the new last node
- increase the stack's length by 1
- return the new node

---

## Example

```js
// current stack:
A <== B (last)

// desired stack:
A <== B        <== C (last)
```

Steps:

```js
// current stack:
A <== B (last)

// set the current last node as the new node's next node
A <== B (last) <== C

// set the new node as the new last node
A <== B        <== C (last)

// desired stack:
A <== B        <== C (last)
```

=> stack after last step equals the desired stack

---

## Implementation

```js
class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class Stack {
  constructor() {
    this.length = 0;
    this.last = null;
  }

  push(value) {
    // create a new node
    const newNode = new Node(value);

    if (!this.length) {
      // stack is empty, therefore set the new node as the last node
      this.last = newNode;
    } else {
      // set the current last node as the new node's next node
      newNode.next = this.last;
      // set the new node as the new last node
      this.last = newNode;
    }

    // increase the stack's length by 1
    this.length += 1;
    // return the new node
    return newNode;
  }
}
```

---

## Result

Let's have a look how to use the `push` method and its results.

```js
const newStack = new Stack();

// should be empty
console.log(newStack);
// Stack { length: 0, last: null }

// one new node
console.log(newStack.push("A"));
// Node { value: 'A', next: null }

// should have one node
console.log(newStack);
// Stack { length: 1, last: Node { value: 'A', next: null } }

// one new node
console.log(newStack.push("B"));
// Node { value: 'B', next: Node { value: 'A', next: null } }

// should have two nodes
console.log(newStack);
// Stack {
//   length: 2,
//   last: Node { value: 'B', next: Node { value: 'A', next: null } }
// }
```

---

## Next Part

We will implement our next method to `pop` the last node.

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