# JavaScript Data Structures: Singly Linked List: Insert

## Intro

Last time, we learned how to update/set a specific node.

Today, we learn how to insert a new node at any specific index.

## Current Code

We start with code that has `push`, `unshift` and `get`, because we can reuse these methods.

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

constructor() {
this.length = 0;
this.tail = null;
}

push(value) {
const newNode = new Node(value);
if (!this.length) {
} else {
this.tail.next = newNode;
}

this.tail = newNode;
this.length += 1;
return newNode;
}

unshift(value) {

if (!this.length) {
} else {
}

this.length += 1;

}

get(index) {
if (index < 0 || index >= this.length) {
return null;
} else {
let count = 0;

while (count < index) {
currentNode = currentNode.next;
count += 1;
}

return currentNode;
}
}
}
``````

## Thoughts

First, we should think about the constraints and possibilities:

If we want to add a node "outside" the List (less than 0 or greater than the length of the current List):

• return null

If we want to add a node at the beginning of the List (index is 0):

• we can use our `unshift` method

If we want to add a node at the end of the List:

• we can use our `push` method

All remaining cases:

• create a new node
• put it between the node before the new node's place and the node, that is currently at this place

Example:

• current List: A -> B
• desired List: A -> N -> B

Steps:

• create new node `N`
• point `N`'s `next` to `B`
• point `A`'s `next` to `N`

## Implementation (Short version, DRY)

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

constructor() {
this.length = 0;
this.tail = null;
}

push(value) {
const newNode = new Node(value);
if (!this.length) {
} else {
this.tail.next = newNode;
}

this.tail = newNode;
this.length += 1;
return newNode;
}

unshift(value) {

if (!this.length) {
} else {
}

this.length += 1;

}

get(index) {
if (index < 0 || index >= this.length) {
return null;
} else {
let count = 0;

while (count < index) {
currentNode = currentNode.next;
count += 1;
}

return currentNode;
}
}

insert(index, value) {
// add a node "outside" the List (=> invalid)
if (index < 0 || index > this.length) {
return null;
} else if (index === 0) {
// add a node to the beginning of the List
return this.unshift(value);
} else if (index === this.length) {
// add a node to the end of the List
return this.push(value);
} else {
// get the node before the new node's desired place (because it has to point to the new node soon)
const preDesiredPlace = this.get(index - 1);
// create a new node
const newNode = new Node(value);
// the new node should point to the node, that is currently at the new node's desired place
newNode.next = preDesiredPlace.next;
// the node before the new node's desired place should point to the new node
preDesiredPlace.next = newNode;
// increase the List's length by 1
this.length += 1;
// return the new node
return newNode;
}
}
}
``````

## Result

Let's have a look how to use the Singly Linked List's `insert` method and its results.

``````const newSLL = new SinglyLinkedList();
console.log(newSLL.insert(0, "A"));
// Node { value: 'A', next: null }

console.log(newSLL.insert(1, "B"));
// Node { value: 'B', next: null }

console.log(newSLL);
//   length: 2,
//   head: Node { value: 'A', next: Node { value: 'B', next: null } },
//   tail: Node { value: 'B', next: null }
// }

console.log(newSLL.insert(1, "N (between A and B)"));
// Node {
//   value: 'N (between A and B)',
//   next: Node { value: 'B', next: null }
// }

console.log(newSLL);
//   length: 3,
//     value: 'A',
//     next: Node { value: 'N (between A and B)', next: [Node] }
//   },
//   tail: Node { value: 'B', next: null }
// }
``````

### Next Part

We will implement how to remove a node at a specific index. If you want to be notified, subscribe :)

### Questions

We are doing it like:

1. point `N`'s `next` to `B`
2. point `A`'s `next` to `N`

What would happen, if we'd switch these steps? 