krahets/hello-algo · error · Error

Illegal Argument Exception

Error message

Illegal Argument Exception

What it means

Thrown by GraphAdjList.addEdge (JS) when an edge cannot be added. The method validates that both endpoints exist in the adjacency list and are distinct vertices before mutating state. This is the standard precondition guard for undirected-graph edge insertion in the hello-algo educational library.

Source

Thrown at ru/codes/javascript/chapter_graph/graph_adjacency_list.js:37

            this.addVertex(edge[0]);
            this.addVertex(edge[1]);
            this.addEdge(edge[0], edge[1]);
        }
    }

    /* Получить число вершин */
    size() {
        return this.adjList.size;
    }

    /* Добавление ребра */
    addEdge(vet1, vet2) {
        if (
            !this.adjList.has(vet1) ||
            !this.adjList.has(vet2) ||
            vet1 === vet2
        ) {
            throw new Error('Illegal Argument Exception');
        }
        // Добавить ребро vet1 - vet2
        this.adjList.get(vet1).push(vet2);
        this.adjList.get(vet2).push(vet1);
    }

    /* Удаление ребра */
    removeEdge(vet1, vet2) {
        if (
            !this.adjList.has(vet1) ||
            !this.adjList.has(vet2) ||
            vet1 === vet2 ||
            this.adjList.get(vet1).indexOf(vet2) === -1
        ) {
            throw new Error('Illegal Argument Exception');
        }
        // Удалить ребро vet1 - vet2
        this.adjList.get(vet1).splice(this.adjList.get(vet1).indexOf(vet2), 1);

View on GitHub (pinned to 69932aed18)

Solutions

1. Ensure every vertex passed to addEdge was first added with addVertex(vet).
2. Guard the call: if (g.adjList.has(v1) && g.adjList.has(v2) && v1 !== v2) g.addEdge(v1, v2).
3. If self-loops are legitimately needed, fork the class or relax the vet1 === vet2 check.
4. When loading a graph from edges, add all distinct vertices in a first pass, then add edges in a second pass.

Example fix

// before
const g = new GraphAdjList();
g.addEdge(v1, v2); // throws if v1/v2 absent

// after
const g = new GraphAdjList();
g.addVertex(v1); g.addVertex(v2);
if (v1 !== v2) g.addEdge(v1, v2);

// Verify both vertices exist and are distinct before addEdge
function canAddEdge(g, v1, v2) {
  return g.adjList.has(v1) && g.adjList.has(v2) && v1 !== v2;
}
if (canAddEdge(g, v1, v2)) g.addEdge(v1, v2);

// Vertex is typically { val: number } in hello-algo
function isVertex(v) {
  return v != null && typeof v === 'object' && 'val' in v;
}

When it happens

Trigger: addEdge(vet1, vet2) is called where vet1 or vet2 was never registered via addVertex (so adjList.has() returns false), or where vet1 === vet2 (a self-loop, which this implementation forbids).

Common situations: Forgetting to call addVertex on a vertex before wiring edges; passing the same Vertex object for both args; reusing a vertex reference after it was removed; building a graph from parsed data without pre-populating the vertex set.

Related errors

• Illegal Argument Exception
• Index Out Of Bounds Exception
• Illegal Argument Exception
• Illegal Argument Exception
• Illegal Argument Exception

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/e08738e592c27d5b. Report an issue: GitHub.