The push method is a bit more involved.

• First, given a value, we want to create a ListNode with that value, referenced by newNode.
• Next, we must consider two scenarios: an empty stack and a non-empty stack.
  • If the stack has no elements, directly assign newNode as the top node.
  • If the stack is not empty:
    • Set the next property of newNode to point to the current top node.
    • Update the top pointer to reference newNode.

Assuming we have created a stack, let's see how adding the first three elements would look visually:

myStack.push(1)

myStack.push(2)

myStack.push(3)

// some code above

push(value) {
  const newNode = new ListNode(value);
  if (!this.top) {
    this.top = newNode;
  } else {
      newNode.next = this.top;
      this.top = newNode;
  }
}

We can simplify this implementation significantly. Remember that our ListNode class allows us to pass in a next value when we create a new node. As a result, we can assign the current top to our new node's next value, handling both scenarios with one action:

// some code above

push(value) {
  this.top = new ListNode(value, this.top);
}

If the stack is empty, this.top is null, which is the default value that ListNode uses when we don't pass in a next argument. If the stack is not empty, this.top references the node that should be referenced by newNode's next pointer.