Intro

Last time, we learned how to pop a new node from the end of our Singly Linked List.

Today, we learn how to unshift something to the list.Unshift meansadd something to the beginning.

Current Code

We start with the codefrom the setup, withoutpush andpop, because we want to keep the code as simple as possible to understand.

classNode{constructor(value){this.value=value;this.next=null;}}classSinglyLinkedList{constructor(){this.length=0;this.head=null;this.tail=null;}}

Thoughts

First, we should think about the constraints and possibilities:

If there is currently NO other node in the Singly Linked List (so it is currently empty):

• create a new node
• set the new node as the Singly Linked List'stail
• set the new node as the Singly Linked List'shead
• increase the Singly Linked List's length by 1
• return the new node

If there is at least 1 node in the Singly Linked List:

• create a new node
• set the new node'snext to the Singly Linked List's currenthead
• set the new node as the Singly Linked List'shead
• increase the Singly Linked List's length by 1
• return the new node

Examples:

• 0 nodes: before: null (head & tail) => after: A (head & tail)
• 1 node: before: A (head & tail) => after: A-1 (head) -> A (tail)
• n nodes: before: A (head) -> ... -> n (tail) => after: A-1 (head) -> A -> ... -> n (tail)

Differences:

• there is only one difference: the step after creating a new node

Implementation (Short version, DRY)

classNode{constructor(value){this.value=value;this.next=null;}}classSinglyLinkedList{constructor(){this.length=0;this.head=null;this.tail=null;}unshift(value){// create a new nodeconstnewNode=newNode(value);// check if Singly Linked List is emptyif(!this.length){// set the new node as the Singly Linked List's `tail`this.tail=newNode;}else{// set the new node's `next` to the Singly Linked List's current `head`newNode.next=this.head;}// set the new node as the Singly Linked List's `head`this.head=newNode;// increase the Singly Linked List's length by 1this.length+=1;// return the new nodereturnnewNode;}}

Result

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

constnewSLL=newSinglyLinkedList();// should be emptyconsole.log(newSLL);// SinglyLinkedList { length: 0, head: null, tail: null }console.log(newSLL.unshift("1"));// Node { value: '1', next: null }// should be a list with the new node with value 1console.log(newSLL);/* *  SinglyLinkedList { *    length: 1, *    head: Node { value: '1', next: null }, *    tail: Node { value: '1', next: null } *  } */console.log(newSLL.unshift("2"));// Node { value: '2', next: Node { value: '1', next: null } }// should be a list with the new node with value 2 and 1 (from the last unshift)console.log(newSLL);/* *  SinglyLinkedList { *    length: 2, *    head: Node { value: '2', next: Node { value: '1', next: null } }, *    tail: Node { value: '1', next: null } *  } */

Next Part

We will implement how to remove a node from the beginning of the Singly Linked List. If you want to be notified,subscribe :)

Questions:

• Any ideas how to improve the post or code?
• Any specific questions?
• Do you like the series or is it useless? Why?