krahets/hello-algo · error · Error

Illegal Argument Exception

Error message

Illegal Argument Exception

What it means

Thrown by addEdge(vet1, vet2) in an adjacency-list graph when either vertex is absent from the Map, or when vet1 === vet2 (self-loops disallowed). Map keys use reference identity (SameValueZero), so a vertex object with the same val but a different reference is treated as absent. The guard prevents calling .push on an undefined adjacency list.

Solutions

1. Add both vertices via addVertex (or the constructor edges) before calling addEdge, using the same object references.
2. Guard the call: if (adjList.has(vet1) && adjList.has(vet2) && vet1 !== vet2) graph.addEdge(vet1, vet2);
3. If val-based identity is needed, key the Map by a primitive id instead of the Vertex object.
4. Wrap in try/catch when adding edges from untrusted sources.

Example fix

// before
graph.addEdge(new Vertex(1), new Vertex(2)); // refs not stored -> throws

// after
const v1 = new Vertex(1), v2 = new Vertex(2);
graph.addVertex(v1);
graph.addVertex(v2);
graph.addEdge(v1, v2);

if (
    graph.adjList.has(vet1) &&
    graph.adjList.has(vet2) &&
    vet1 !== vet2
) {
    graph.addEdge(vet1, vet2);
}

function canAddEdge(graph, vet1, vet2) {
    return (
        graph.adjList.has(vet1) &&
        graph.adjList.has(vet2) &&
        vet1 !== vet2
    );
}

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

When it happens

Trigger: Calling addEdge with a Vertex not previously added via the constructor or addVertex; passing two different references that represent the same logical vertex; attempting addEdge(v, v) to form a self-loop.

Common situations: Deserializing vertices from storage and passing new instances to addEdge; aliasing bugs where two Vertex objects share a val but differ in reference; graph algorithms that try self-loops.

Related errors

• Illegal Argument Exception
• Illegal Argument Exception
• error
• error
• Illegal Argument Exception

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/afec66320b8ce99f. Report an issue: GitHub.

Appendix: source

Thrown at zh-hant/codes/javascript/chapter_graph/graph_adjacency_list.js:37

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

    /* 獲取頂點數量 */
    size() {
        return this.adjList.size;
    }

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

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

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