krahets/hello-algo · error · Error

Illegal Argument Exception

Error message

Illegal Argument Exception

What it means

Thrown by addEdge(vet1, vet2) on a graph represented as an adjacency list (Map of vertex -> neighbor array). The method refuses the operation if either vertex is absent from adjList, or if vet1 === vet2 (self-loops are disallowed in this undirected graph model). It then pushes each vertex into the other's neighbor list, so a missing vertex would cause an undefined.push() crash without the guard.

Source

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

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

    /* 获取顶点数量 */
    size() {
        return this.adjList.size;
    }

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

    /* 删除边 */
    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');
        }
        // 删除边 vet1 - vet2
        this.adjList.get(vet1).splice(this.adjList.get(vet1).indexOf(vet2), 1);

View on GitHub (pinned to 69932aed18)

Solutions

1. Ensure both endpoints are added first: if (!g.adjList.has(a)) g.addVertex(a); repeat for b; then g.addEdge(a,b).
2. Do not pass the same reference for both args; if self-loops are needed, extend the class rather than bypassing the check.
3. Use the exact object references returned/stored by addVertex — Map key equality is by reference for objects.
4. Validate input data before constructing the graph so no orphan edges exist.

Example fix

// before
const a = { val: 1 }, b = { val: 2 };
g.addEdge(a, b); // throws if a/b not added
// after
g.addVertex(a); g.addVertex(b);
g.addEdge(a, b);

function safeAddEdge(g, a, b) {
  if (!g.adjList.has(a)) g.addVertex(a);
  if (!g.adjList.has(b)) g.addVertex(b);
  if (a !== b) g.addEdge(a, b);
}

function isRegisteredVertex(g, v) {
  return g.adjList.has(v);
}

try {
  g.addEdge(a, b);
} catch (e) {
  if (e instanceof Error && e.message === 'Illegal Argument Exception') {
    // ensure vertices exist and a !== b, then retry or skip
  } else throw e;
}

When it happens

Trigger: Calling addEdge before both vertices were addVertex()'d; passing the same object reference for both arguments (self-loop); passing a primitive that was never registered; passing a vertex from a different GraphAdjList instance.

Common situations: Building edges before nodes; assuming vertices auto-create on first edge; reusing object literals whose reference identity differs from the stored vertex (Map keys compare by reference); loading graph data where some node rows are missing.

Related errors

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

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/3d7a3a592942427d. Report an issue: GitHub.