为什么我收到一条错误消息，提示“ TypeError：无法读取未定义的属性'execute'”

Question

我的问题是我的代码中有client.commands.get('help').execute(message, args, version, PREFIX);的许多变体，但是在这种情况下（第43行）我得到一个错误。这是我的代码。

const client = new Discord.Client();
const token = "Njg5MTcyMzc4NTcxMzc0NzE3.Xm_APA.EcT6EZS5OnhzLA3fUbddsfI63Xw";
const PREFIX = ";";
const version = 'Alpha 1.0.1'

const fs = require('fs');
client.commands = new Discord.Collection();

const commandFiles = fs.readdirSync('./commands/').filter(file => file.endsWith('.js'));
for(const file of commandFiles){
    const command = require(`./commands/${file}`);

    client.commands.set(command.name, command);
}

client.on('ready', () => {
    console.log("Ready!");
});

client.on('message', message => {
    let args = message.content.substring(PREFIX.length).split(" ");

    switch (args[0]) {
        case 'test':
            client.commands.get('test').execute(message, args, version);
            break;
        case 'help':
            client.commands.get('help').execute(message, args, version, PREFIX);
            break;
        case 'version':
            message.channel.send('Version: ' + version)
            break;
        case 'convoy':
            if (args[1] = 'sim1') {
                var server = 'Simulation 1 in ETS2'
            } else if (args[1] = 'promods') {
                var server = 'ProMods 1 in ETS2'
            } else if (args[1] = 'us') {
                var server = '[US] Simulation in ATS'
            }
            client.commands.get('convoy').execute(message, args, server);
            break;

        }
});

client.login(token);```