Understanding Graphs in JavaScript: Adjacency Matrix vs. Adjacency List

Introduction

Graphs are one of the most fundamental data structures in computer science. They consist of nodes (also called vertices) connected by edges. Graphs are widely used in various real-world applications, such as social networks, navigation systems, and recommendation engines.

In this post, we'll explore two common ways to represent graphs: Adjacency Matrix and Adjacency List. We'll also implement both representations in JavaScript and compare their efficiency.

Types of Graph Representations

1. Adjacency Matrix

An adjacency matrix is a 2D array (or matrix) that represents connections between nodes. If there is an edge between node i and node j, the matrix at position [i][j] contains 1 (or the weight of the edge, in case of weighted graphs). Otherwise, it contains 0.

Implementation in JavaScript

class Graph {
    constructor(numNodes) {
        this.matrix = [];
        for (let i = 0; i < numNodes; i++) {
            this.matrix.push(new Array(numNodes).fill(0));
        }
    }

    addEdge(to, from) {
        if (!this.isEdge(to, from) && !this.isEdge(from, to)) {
            this.matrix[to][from] = 1;
            this.matrix[from][to] = 1;
        } else {
            console.log("Edge already exists", to, from);
        }
    }

    removeEdge(from, to) {
        this.matrix[from][to] = 0;
        this.matrix[to][from] = 0;
    }

    isEdge(from, to) {
        return this.matrix[from][to] === 1;
    }
}

const graph = new Graph(3);
graph.addEdge(0, 1);
graph.addEdge(0, 2);
console.log(graph.matrix);

/*
Output : 
[
  [0, 1, 1],
  [1, 0, 0],
  [1, 0, 0]
]
*/

2. Adjacency List

An adjacency list represents a graph using a hash map (object in JavaScript) where each key (node) stores an array of connected nodes.

Implementation in JavaScript

// adjacency list ( unweighted and undirected graph )
/**
 * {
  1: [2, 3],
  2: [1],
  3: [1]
}
 */

class AdjacencyList {
    constructor() {
        this.list = {}
    }

    addNode(node) {
        if(this.list[node]){
            console.log(`${node} already exists`);
            return false; 
        }
        this.list[node] = []
        return true
    }

    addEdge(from, to) {
        if (!this.list[from] || !this.list[to]) {
            console.log(`One or both nodes (${from}, ${to}) do not exist.`);
            return;
        }

        if(!this.isEdge(from, to)){
            this.list[from].push(to);
            this.list[to].push(from);
        }
    }

    removeEdge(from, to) {
        if (this.list[from]) {
            this.list[from] = this.list[from].filter(node => node !== to)
        }

        if (this.list[to]) {
            this.list[to] = this.list[to].filter(node => node !== from)
        }
    }

    isEdge(from, to) {
        //return this.list[from]?.includes(to) || false;
        return this.list[from] ? this.list[from].indexOf(to) !== -1 : false
    }
}

const newGraph = new AdjacencyList();
newGraph.addNode(1)
newGraph.addNode(2)
newGraph.addNode(3)

newGraph.addEdge(1,2)
newGraph.addEdge(1,3)

console.log(newGraph)

Adjacency Matrix vs. Adjacency List - Edge Checking Differences

Adjacency Matrix (2D Array)

if (!this.isEdge(from, to) || !this.isEdge(to, from))

• The matrix is a fixed-size 2D array where we manually set both matrix[from][to] and matrix[to][from] to 1.

• Since we store edges explicitly in both directions, checking both is necessary to ensure symmetry in an undirected graph.

• Example Matrix for 3 nodes:

    0  1  2
  0 [0, 1, 1]
  1 [1, 0, 0]
  2 [1, 0, 0]
  

  • If (0,1) exists, then (1,0) must also exist.

  • The check ensures both positions are updated consistently.

Adjacency List (Object with Arrays)

if (!this.isEdge(from, to)) { 

• The adjacency list stores edges in a single-directional way (we manually push both from → to and to → from).

• If from contains to, then to will automatically contain from, because we add both at the same time.

• Example List:

  {
      0: [1, 2],
      1: [0],
      2: [0]
  }
  

  • If 0 → 1 exists, then 1 → 0 is already guaranteed.

  • So, checking just one direction is enough.

Real-World Applications of Graphs

• Social Networks (Facebook friends, LinkedIn connections)

• Google Maps (Finding the shortest route)

• Recommendation Systems (Netflix, Amazon product suggestions)

Conclusion

Both Adjacency Matrix and Adjacency List have their pros and cons. The choice depends on the use case:

• Use Adjacency Matrix for dense graphs where edge lookup speed is crucial.

• Use Adjacency List for sparse graphs where memory optimization matters.

Which approach do you prefer? Let me know in the comments! 🚀

Thank you for reading! I hope this gave you a clear understanding of adjacency matrices and lists. In the next blog, we’ll dive into graph traversal techniques like BFS and DFS.

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