LinkedList Implementation in JavaScript

In this article, we will learn how to implement a LinkedList in JavaScript.

The Linked List Data Structure Overview

Linked lists store a sequential collection of elements; but unlike arrays, in linked lists, the elements are not placed contiguously in memory. Each element consists of a node that stores the element itself and also a reference (also known as a pointer or link) that points to the next element. The following diagram exemplifies the structure of a linked list:

Linked List Implementation Using JavaScript

Let's first create a Node class:
class Node {
  constructor(element, next) {
    this.element = element;
    this.next = next;
  }
}
Now, let's create a Linked List implementation in JavaScript:
class LinkedList {
    constructor(equalsFn = defaultEquals) {
        this.equalsFn = equalsFn;
        this.count = 0;
        this.head = undefined;
    }

    push(element) {
        const node = new Node(element);
        let current;
        if (this.head == null) {
            // catches null && undefined
            this.head = node;
        } else {
            current = this.head;
            while (current.next != null) {
                current = current.next;
            }
            current.next = node;
        }
        this.count++;
    }

    getElementAt(index) {
        if (index >= 0 && index <= this.count) {
            let node = this.head;
            for (let i = 0; i < index && node != null; i++) {
                node = node.next;
            }
            return node;
        }
        return undefined;
    }

    insert(element, index) {
        if (index >= 0 && index <= this.count) {
            const node = new Node(element);
            if (index === 0) {
                const current = this.head;
                node.next = current;
                this.head = node;
            } else {
                const previous = this.getElementAt(index - 1);
                node.next = previous.next;
                previous.next = node;
            }
            this.count++;
            return true;
        }
        return false;
    }

    removeAt(index) {
        if (index >= 0 && index < this.count) {
            let current = this.head;
            if (index === 0) {
                this.head = current.next;
            } else {
                const previous = this.getElementAt(index - 1);
                current = previous.next;
                previous.next = current.next;
            }
            this.count--;
            return current.element;
        }
        return undefined;
    }

    remove(element) {
        const index = this.indexOf(element);
        return this.removeAt(index);
    }

    indexOf(element) {
        let current = this.head;
        for (let i = 0; i < this.size() && current != null; i++) {
            if (this.equalsFn(element, current.element)) {
                return i;
            }
            current = current.next;
        }
        return -1;
    }

    isEmpty() {
        return this.size() === 0;
    }

    size() {
        return this.count;
    }

    getHead() {
        return this.head;
    }

    clear() {
        this.head = undefined;
        this.count = 0;
    }

    toString() {
        if (this.head == null) {
            return '';
        }
        let objString = `${this.head.element}`;
        let current = this.head.next;
        for (let i = 1; i < this.size() && current != null; i++) {
            objString = `${objString},${current.element}`;
            current = current.next;
        }
        return objString;
    }
}

function defaultEquals(a, b) {
    return a === b;
}

Using the Linked List

const list = new LinkedList();

console.log('push element 15');
list.push(15);

console.log('list.indexOf(15) => ', list.indexOf(15));

console.log('push element 10');
list.push(10);

console.log('list.toString() => ', list.toString());
console.log('list.indexOf(10) => ', list.indexOf(10));

console.log('push element 13');
list.push(13);

console.log('list.toString() => ', list.toString());
console.log('list.indexOf(13) => ', list.indexOf(13));
console.log('list.indexOf(10) => ', list.indexOf(10));

console.log('push elements 11 and 12');
list.push(11);
list.push(12);

console.log('list.toString() => ', list.toString());
console.log('list.removeAt(1) => ', list.removeAt(1));
console.log('list.toString() => ', list.toString());
console.log('list.removeAt(3) => ', list.removeAt(3));
console.log('list.toString() => ', list.toString());

console.log('push element 14');
list.push(14);

console.log('list.toString() => ', list.toString());
console.log('insert element 16 pos 0 => ', list.insert(16, 0));
console.log('list.toString() => ', list.toString());
console.log('insert element 17 pos 1 => ', list.insert(17, 1));
console.log('list.toString() => ', list.toString());
console.log('insert element 18 pos list.size() => ', list.insert(18, list.size()));
console.log('list.toString() => ', list.toString());
console.log('remove element 16 => ', list.remove(16));
console.log('list.toString() => ', list.toString());
console.log('remove element 11 => ', list.remove(11));
console.log('list.toString() => ', list.toString());
console.log('remove element 18 => ', list.remove(18));
console.log('list.toString() => ', list.toString());

Output

push element 15
list.indexOf(15) =>  0
push element 10
list.toString() =>  15,10
list.indexOf(10) =>  1
push element 13
list.toString() =>  15,10,13
list.indexOf(13) =>  2
list.indexOf(10) =>  1
push elements 11 and 12
list.toString() =>  15,10,13,11,12
list.removeAt(1) =>  10
list.toString() =>  15,13,11,12
list.removeAt(3) =>  12
list.toString() =>  15,13,11
push element 14
list.toString() =>  15,13,11,14
insert element 16 pos 0 =>  true
list.toString() =>  16,15,13,11,14
insert element 17 pos 1 =>  true
list.toString() =>  16,17,15,13,11,14
insert element 18 pos list.size() =>  true
list.toString() =>  16,17,15,13,11,14,18
remove element 16 =>  16
list.toString() =>  17,15,13,11,14,18
remove element 11 =>  11
list.toString() =>  17,15,13,14,18
remove element 18 =>  18
list.toString() =>  17,15,13,14

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