A Guide to Discord Bots

Prefix & More Commands

Prefix

A prefix is something that you attach before every command.
Most bots have one, it helps to reduce RAM usage and you can have multiple bots with the sames commands, but a different prefix.
In this book, I'll use the prefix *.

Let's implement it into our code:

client.on('message', message => {
  if (message.channel.type != 'text' || message.author.bot || !message.startsWith('*'))
    return;

  if (message.content === '*ping') {
    message.channel.send('pong');
  }
});

The startsWith() method.
!message.startsWith('*') will make sure the bot ignores the message if it doesn't start with our prefix.
I also edited the condition to check if the message is *ping.

More commands

Let's add a *say command!
This command will make the bot say what we want. Simple, right?

client.on('message', message => {
  if (message.channel.type != 'text' || message.author.bot || !message.startsWith('*'))
    return;

  if (message.content === '*ping') {
    message.channel.send('pong');
  }

  else if (message.content === '*say') {
    message.channel.send(message.content);
  }
});

Message.content

Let's test it!
... It only answers with *say when I type *say and does nothing else... why?

Because your bot is checking if the message is exactly *say.
To fix that, we can use, as seen earlier, message.content.startsWith('*say');.
It works! ...almost.
It still answers with *say + what you typed.

One solution could be:

else if (message.content.startsWith('*say')) {
    /* this will make sure the person didn't just type '*say',
       but something like '*say hi'... */
    if (!message.content.includes(' '))
      return;

    /* ...or else this would make your bot crash.
      this will only remove the first '*say ' found */
    message.channel.send(message.content.replace('*say ', '');
  }

Of course, we don't necessarily need the check, because we could find a better solution, but the reason why I included it is because we can make the bot answer with the command's syntax.
Example: Syntax: *say <message>.

If...else if...else if...switch?

Having multiple else if statements is not really a bad thing, but you might be interesting in using a switch instead.
If you have many else if, using a switch statement is often faster.
Note: you'll need a better way to determine which command was used.

let command = message.content.split(' ')[0].slice(1);
/* message.content: '*say hello world'
   message.content.split(' '): ['*say', 'hello', 'world']
   message.content.split(' ')[0]: '*say'
   message.content.split(' ')[0].slice(1): 'say'
*/

// Arguments will be useful in the next part
let args = message.content.replace('*' + command, '').trim();

switch (command) {
  case 'ping':
    message.channel.send('pong');
    break; /* <- don't forget this!
    If you don't put it, it will execute the next command too */

  case 'pong':
    // You don't need brackets inside a switch
    message.channel.send('ping');
    break;

  case 'say': {
    // But you can use them to avoid a conflict between let statements
    let sentence = message.content.replace(command, '').trim();
    message.channel.send(sentence);
    message.delete(); /* not obligatory,
    don't forget you can customize your commands :) */
    break;
  }

  case 'test': {
    /* 'let sentence' is not already defined, because we put brackets
       in the previous command */
    let sentence = 'brackets are useful';
    console.log(sentence);
    break; /* <- useless for the last command, but as you add more,
    you might forget to add it*/
  }
}

trim() will remove whitespaces from the left and the right of the given string.
You can alternatively use trimLeft() or trimRight(), but trim() is more common.