# JavaScript Data Structures - Graph

Angelos Chalaris · Aug 17, 2021 ·

JavaScript, Object, Class, Array ### Definition

A graph is a data structure consisting of a set of nodes or vertices and a set of edges that represent connections between those nodes. Graphs can be directed or undirected, while their edges can be assigned numeric weights. Each node in a graph data structure must have the following properties:

• `key`: The key of the node
• `value`: The value of the node

Each edge in a grpah data structure must have the following properties:

• `a`: The starting node of the edge
• `b`: The target node of the edge
• `weight`: An optional numeric weight value for the edge

The main operations of a graph data structure are:

• `addNode`: Inserts a new node with the specific key and value
• `addEdge`: Inserts a new edge between two given nodes, optionally setting its weight
• `removeNode`: Removes the node with the specified key
• `removeEdge`: Removes the edge between two given nodes
• `findNode`: Retrieves the node with the given key
• `hasEdge`: Checks if the graph has an edge between two given nodes
• `setEdgeWeight`: Sets the weight of a given edge
• `getEdgeWeight`: Gets the weight of a given edge
• `adjacent`: Finds all nodes for which an edge exists from a given node
• `indegree`: Calculates the total number of edges to a given node
• `outdegree`: Calculates the total number of edges from a given node

### Implementation

```class Graph {
constructor(directed = true) {
this.directed = directed;
this.nodes = [];
this.edges = new Map();
}

this.nodes.push({ key, value });
}

this.edges.set(JSON.stringify([a, b]), { a, b, weight });
if (!this.directed)
this.edges.set(JSON.stringify([b, a]), { a: b, b: a, weight });
}

removeNode(key) {
this.nodes = this.nodes.filter(n => n.key !== key);
[...this.edges.values()].forEach(({ a, b }) => {
if (a === key || b === key) this.edges.delete(JSON.stringify([a, b]));
});
}

removeEdge(a, b) {
this.edges.delete(JSON.stringify([a, b]));
if (!this.directed) this.edges.delete(JSON.stringify([b, a]));
}

findNode(key) {
return this.nodes.find(x => x.key === key);
}

hasEdge(a, b) {
return this.edges.has(JSON.stringify([a, b]));
}

setEdgeWeight(a, b, weight) {
this.edges.set(JSON.stringify([a, b]), { a, b, weight });
if (!this.directed)
this.edges.set(JSON.stringify([b, a]), { a: b, b: a, weight });
}

getEdgeWeight(a, b) {
return this.edges.get(JSON.stringify([a, b])).weight;
}

return [...this.edges.values()].reduce((acc, { a, b }) => {
if (a === key) acc.push(b);
return acc;
}, []);
}

indegree(key) {
return [...this.edges.values()].reduce((acc, { a, b }) => {
if (b === key) acc++;
return acc;
}, 0);
}

outdegree(key) {
return [...this.edges.values()].reduce((acc, { a, b }) => {
if (a === key) acc++;
return acc;
}, 0);
}
}```
• Create a `class` with a `constructor` that initializes an empty array, `nodes`, and a `Map`, `edges`, for each instance. The optional argument, `directed`, specifies if the graph is directed or not.

• Define an `addNode()` method, which uses `Array.prototype.push()` to add a new node in the `nodes` array.

• Define an `addEdge()` method, which uses `Map.prototype.set()` to add a new edge to the `edges` Map, using `JSON.stringify()` to produce a unique key.

• Define a `removeNode()` method, which uses `Array.prototype.filter()` and `Map.prototype.delete()` to remove the given node and any edges connected to it.

• Define a `removeEdge()` method, which uses `Map.prototype.delete()` to remove the given edge.

• Define a `findNode()` method, which uses `Array.prototype.find()` to return the given node, if any.

• Define a `hasEdge()` method, which uses `Map.prototype.has()` and `JSON.stringify()` to check if the given edge exists in the `edges` Map.

• Define a `setEdgeWeight()` method, which uses `Map.prototype.set()` to set the weight of the appropriate edge, whose key is produced by `JSON.stringify()`.

• Define a `getEdgeWeight()` method, which uses `Map.prototype.get()` to get the eight of the appropriate edge, whose key is produced by `JSON.stringify()`.

• Define an `adjacent()` method, which uses `Map.prototype.values()`, `Array.prototype.reduce()` and `Array.prototype.push()` to find all nodes connected to the given node.

• Define an `indegree()` method, which uses `Map.prototype.values()` and `Array.prototype.reduce()` to count the number of edges to the given node.

• Define an `outdegree()` method, which uses `Map.prototype.values()` and `Array.prototype.reduce()` to count the number of edges from the given node.

```const g = new Graph();

g.nodes.map(x => x.value);  // ['a', 'b', 'c', 'd']
[...g.edges.values()].map(({ a, b }) => `\${a} => \${b}`);
// ['a => c', 'b => c', 'c => b', 'd => a']

g.indegree('c');            // 2
g.outdegree('c');           // 1

g.hasEdge('d', 'a');        // true
g.hasEdge('a', 'd');        // false

g.removeEdge('c', 'b');

[...g.edges.values()].map(({ a, b }) => `\${a} => \${b}`);
// ['a => c', 'b => c', 'd => a']

g.removeNode('c');

g.nodes.map(x => x.value);  // ['a', 'b', 'd']
[...g.edges.values()].map(({ a, b }) => `\${a} => \${b}`);
// ['d => a']

g.setEdgeWeight('d', 'a', 5);
g.getEdgeWeight('d', 'a');  // 5```

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