## Intro

Last time, 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

``````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

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

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

Steps:

``````// 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

``````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.

``````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.

If you want to get notified, subscribe!