krahets/hello-algo · error · Error

Illegal Argument Exception

Error message

Illegal Argument Exception

What it means

Thrown by the adjacency-list graph's addEdge method when an edge cannot be safely created. The guard rejects three cases: either endpoint vertex is not registered in the Map, or both arguments are the exact same vertex reference (self-loops are disallowed). This is a defensive precondition check so the subsequent adjList.get(...).push(...) calls never receive undefined.

Source

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

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

    /* 頂点数を取得 */
    size() {
        return this.adjList.size;
    }

    /* 辺を追加 */
    addEdge(vet1, vet2) {
        if (
            !this.adjList.has(vet1) ||
            !this.adjList.has(vet2) ||
            vet1 === vet2
        ) {
            throw new Error('Illegal Argument Exception');
        }
        // 辺 vet1 - vet2 を追加
        this.adjList.get(vet1).push(vet2);
        this.adjList.get(vet2).push(vet1);
    }

    /* 辺を削除 */
    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');
        }
        // 辺 vet1 - vet2 を削除
        this.adjList.get(vet1).splice(this.adjList.get(vet1).indexOf(vet2), 1);

View on GitHub (pinned to 69932aed18)

Solutions

1. Call addVertex(vet1) and addVertex(vet2) for both endpoints before calling addEdge, and pass back the exact references the graph holds.
2. Ensure vet1 and vet2 are distinct object references (vet1 !== vet2) before calling addEdge.
3. If you construct vertices from numeric values, maintain your own id->Vertex map so you always retrieve the stored reference instead of constructing a duplicate.
4. Wrap the call in try/catch only when the inputs are genuinely untrusted.

Example fix

// before
const v = new Vertex(1);
graph.addEdge(v, other);  // v was never added -> Illegal Argument Exception

// after
const v = new Vertex(1);
graph.addVertex(v);
graph.addEdge(v, other);

// Before addEdge, confirm both endpoints are registered and distinct.
function safeAddEdge(graph, vet1, vet2) {
  if (vet1 === vet2) return false;
  if (!graph.adjList.has(vet1) || !graph.adjList.has(vet2)) return false;
  graph.addEdge(vet1, vet2);
  return true;
}

// Vertex membership guard (Map uses reference equality).
const isRegisteredVertex = (graph, v) => graph.adjList.has(v);

try {
  graph.addEdge(vet1, vet2);
} catch (e) {
  if (e instanceof Error && e.message === 'Illegal Argument Exception') {
    // handle: missing vertex or self-loop
  } else throw e;
}

When it happens

Trigger: Calling addEdge(vet1, vet2) when vet1 or vet2 was never passed to addVertex (so adjList.has(...) returns false); passing the same object reference for both arguments (vet1 === vet2); passing a freshly-constructed Vertex whose reference differs from the one stored in the graph (Map keys use SameValueZero/reference equality).

Common situations: Building a graph from raw values and forgetting to addVertex first; creating a new Vertex(val) at call time instead of reusing the stored instance; copy-pasting addEdge calls into a loop where one operand is reused as both endpoints.

Related errors

• Illegal Argument Exception
• Illegal Argument 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/fc1cfc36c913e10d. Report an issue: GitHub.