Node.js开发WhatsApp机器人遇报错：client未初始化就绑定事件

我正在使用whatsapp-web-js包开发WhatsApp聊天机器人，但作为Node.js及异步代码新手，遇到了实现问题。

原代码 #

let client;
    
//Connect to the database to get old persisted session data 

mongoose.connect('mongodb+srv://...url').then(() => {
    //once connected, create new session "client" with persisted data
    const store = new MongoStore({ mongoose: mongoose });
    client = new Client({
        authStrategy: new RemoteAuth({
            store: store,
            backupSyncIntervalMs: 60000
        })
    });
    ready = false;
    client.initialize().then(r => console.log("Initializing whatsapp client"));

    //Generate QR Code if needed
    const qrcode = require('qrcode-terminal');
    client.on('qr', qr => {
        qrcode.generate(qr, {small: true});
    });
});

//Client has saved remote session
client.on('remote_session_saved', () => {
    console.log('Client remote session saved');
});

//Client is ready to send/receive requests
client.on('ready', () => {
    console.log('Client is ready to send/receive requests');
});

问题原因 #

触发空指针错误的核心是Node.js的异步执行特性：mongoose.connect()是异步操作，会在后台执行数据库连接，而主线程会继续执行后续绑定事件的代码——此时client还未被初始化，处于undefined状态，自然会报错。

解决方法 #

把所有需要操作client的代码（包括事件监听）都放到mongoose.connect()的then回调内部，确保只有在数据库连接完成、client实例创建后，再去绑定事件：

const qrcode = require('qrcode-terminal');
let client;
    
//Connect to the database to get old persisted session data 

mongoose.connect('mongodb+srv://...url').then(() => {
    //once connected, create new session "client" with persisted data
    const store = new MongoStore({ mongoose: mongoose });
    client = new Client({
        authStrategy: new RemoteAuth({
            store: store,
            backupSyncIntervalMs: 60000
        })
    });
    ready = false;
    client.initialize().then(r => console.log("Initializing whatsapp client"));

    //Generate QR Code if needed
    client.on('qr', qr => {
        qrcode.generate(qr, {small: true});
    });

    //Client has saved remote session
    client.on('remote_session_saved', () => {
        console.log('Client remote session saved');
    });

    //Client is ready to send/receive requests
    client.on('ready', () => {
        console.log('Client is ready to send/receive requests');
    });
});

注：将require('qrcode-terminal')移到文件顶部，避免在回调内重复加载模块，优化代码结构。

内容的提问来源于stack exchange，提问作者Oirampok