有没有办法在 Node.js 中设置一个可能未定义为小写的变量？

Question

I'm making a discord.js bot that has commands that can have different parameters, such as mc!start vanilla, mc!start tekkit. Only singular string entries are allowed, as that's what I made it to do. But, if the user does not input a parameter, and just does mc!start, I want it to say that you cannot leave the parameter blank, but when I input only mc!start, the script gives me the "TypeError: Cannot read property 'toLowerCase' of undefined" error. I've been trying to do assignment functions and other things like that, but no cigar. Here is a splice of the code.

client.on('message', (message) => {
    const messagearray = message.content.trim().split(' ');
    const command = messagearray[0];
    const minecraft_server = messagearray[1].toLowerCase();
    if(command === (`${prefix}help`)) {
        message.channel.send('Current Commands: !help, !start(Vanilla, Tekkit, Pixelmon, FarmingValley), !shutdown, !randomsong');
    }
    else if(command === (`${prefix}start`)) {
        if(mcVersions.indexOf(minecraft_server) === -1) {
// more code

Answer 1

Check whether the array element is set before trying to call methods of it.

const minecraft_server = messagearray[1] ? messagearray[1].toLowerCase() : "";

Instead of "" at the end you could provide a default server.