working version

.eslintrc.js

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+module.exports = {
+    "extends": [
+        "eslint-config-prettier"
+    ]
+}

.gitignore

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+node_modules

.prettierrc.json

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+{}

package.json

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+{
+  "name": "js-linkedlist",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "repository": {
+    "type": "git",
+    "url": "ssh://_gitea@git.alexanderk.net:3022/ak/js-linkedlist.git"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "devDependencies": {
+    "eslint": "^8.43.0",
+    "eslint-config-prettier": "^8.8.0",
+    "prettier": "^2.8.8"
+  },
+  "type": "module"
+}

src/LinkedList.js

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+import { Node } from "./Node.js";
+
+// linked list factory
+export const LinkedList = () => {
+    const CLU = {
+        firstNode: null,
+        lastNode: null,
+        totalNodes: 0
+    }
+    // adds a new node containing value to the end of the list
+    const append = (value) => {
+        // make node
+        const newNode = Node(value);
+        // set it to first node if we have no others
+        if (CLU.totalNodes === 0) {
+            CLU.firstNode = CLU.lastNode = newNode;
+            CLU.totalNodes++;
+            return;
+        }
+        // else
+        CLU.lastNode.nextNode = CLU.lastNode = newNode;
+        CLU.totalNodes++;
+    }
+    // adds a new node containing value to the start of the list
+    const prepend = (value) => {
+        // set it to first node if we have no others
+        if (CLU.totalNodes === 0) {
+            // make node
+            const newNode = Node(value);
+            CLU.firstNode = CLU.lastNode = newNode;
+            CLU.totalNodes++;
+            return;
+        }
+        // else
+        // make node
+        const newNode = Node(value, CLU.firstNode);
+        CLU.firstNode = newNode;
+        CLU.totalNodes++;
+    }
+    // returns the total number of nodes in the list
+    const size = () => {
+        return CLU.totalNodes;
+    }
+    // returns the first node in the list
+    const head = () => {
+        return CLU.firstNode;
+    }
+    // returns the last node in the list
+    const tail = () => {
+        return CLU.lastNode;
+    }
+    // returns the node at the given index
+    const at = (index) => {
+        // set to first node
+        const MCP = {
+            currentNode: CLU.firstNode
+        }
+        // loop through LL to index
+        for (let i = 0; i < index; i++) {
+            MCP.currentNode = MCP.currentNode.nextNode;
+        }
+        return MCP.currentNode;
+    }
+    // removes the last element from the list
+    const pop = () => {
+        // gets second to last node
+        const penultimateNode = at(CLU.totalNodes - 2);
+        // erases from chain
+        penultimateNode.nextNode = null;
+        // updates node count
+        CLU.totalNodes = CLU.totalNodes - 1;
+        // catches the offchance
+        if (CLU.totalNodes === 0) {
+            CLU.firstNode = CLU.lastNode = null;
+        }
+    }
+    // returns true if the passed in value is in the list and otherwise returns false.
+    const contains = (value) => {
+        find(value) ? true : false;
+    }
+    // returns the index of the node containing value, or null if not found.    !!!!!!!!!!!
+    const find = (value) => {
+        // set to first node
+        const MCP = {
+            currentNode: CLU.firstNode
+        }
+        // for all nodes
+        for (let i = 0; i < CLU.totalNodes; i++) {
+            // if node value matches, set to true
+            if (MCP.currentNode.value === value) {
+                bool = true;
+            }
+            // otherwise set to next node
+            MCP.currentNode = MCP.currentNode.nextNode;
+        }
+        return bool;
+    }
+    // print Nodes out and preview them in the console
+    const toString = () => {
+        // final String value
+        let finalStr = '';
+        // set to first node
+        const MCP = {
+            currentNode: CLU.firstNode
+        }
+        // loop through LL to index
+        for (let i = 0; i < CLU.totalNodes; i++) {
+            finalStr = finalStr + ` ( ${MCP.currentNode.value} ) ->`;
+            MCP.currentNode = MCP.currentNode.nextNode;
+        }
+        finalStr = finalStr + " null";
+        return finalStr;
+    }
+    // inserts a new node with the provided value at the given index
+    const insertAt = (value, index) => {
+        if (index === 0) {
+            prepend(value);
+            return;
+        }
+        if (index === CLU.totalNodes) {
+            append(value);
+            return;
+        }
+        if (index === CLU.totalNodes + 1) {
+            console.log("Your index value is too high!");
+            return;
+        }
+        // gets node at previous index
+        const prevNode = at(index - 1);
+        // gets node at current index
+        const currNode = at(index);
+        // makes new node, sets current node at index as next
+        const newNode = Node(value, currNode);
+        // sets itself as previous index node's next node
+        prevNode.nextNode = newNode;
+        // updates counter
+        CLU.totalNodes = CLU.totalNodes + 1;
+    }
+    // removes the node at the given index
+    const removeAt = (index) => {
+        // pops if final value
+        if (index === CLU.totalNodes - 1) {
+            pop();
+            return;
+        }
+        if (index === 0) {
+            CLU.firstNode = CLU.firstNode.nextNode;
+            CLU.totalNodes = CLU.totalNodes - 1;
+            return;
+        }        
+        // gets node at previous index
+        const prevNode = at(index - 1);
+        // sets its next node to the one ahead of the current one, effectively removing it from the chain
+        prevNode.nextNode = at(index + 1);
+        // removes a digit from the counter
+        CLU.totalNodes = CLU.totalNodes - 1;
+        // catches the offchance
+        if (CLU.totalNodes === 0) {
+            CLU.firstNode = CLU.lastNode = null;
+        }
+    }
+    return {
+        append,
+        prepend,
+        size,
+        head,
+        tail,
+        at,
+        pop,
+        contains,
+        find,
+        toString,
+        insertAt,
+        removeAt
+    }
+}

src/Node.js

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+// node factory
+export const Node = (value = null, nextNode = null) => { 
+    return {
+        value, 
+        nextNode
+    }
+}