我有這兩門(mén)課：class Node {    constructor(nodeId){        this.nodeId = nodeId;        this.adjacencies = [];    }    connectToNode(nodeToConnectTo){        this.adjacencies.push(nodeToConnectTo);    }}class Graph{    constructor(nodes){        this.nodes = nodes;    }    printGraph(){        for (let node in this.nodes){            console.log(node.nodeId);        }            }}我只是想通過(guò)這種方式調(diào)用printGraph打印所有nodeIds ：let node1 = new Node('1');let node2 = new Node('2');let node3 = new Node('3');const arr = [node1, node2, node3];let graph = new Graph(arr);graph.printGraph();但它正在打印undefined。我似乎無(wú)法弄清楚為什么它不簡(jiǎn)單地打印nodeId.