discord.js/docs/examples/commands.md

Slash Commands

In this example, you'll get to know how to create commands and listen to incoming interactions.

Creating a Command

First off, we need to create a command so that users can use it. We will create an echo command which simply returns what the user inputted. Note that global commands can take up to an hour to appear in the client, so if you want to test a new command, you should create it for one guild first.

// The data for our command
const commandData = {
  name: 'echo',
  description: 'Replies with your input!',
  options: [{
    name: 'input',
    type: 'STRING',
    description: 'The input which should be echoed back',
    required: true,
  }],
};

client.once('ready', () => {
  // Creating a global command
  client.application.commands.create(commandData);

  // Creating a guild-specific command
  client.guilds.cache.get('id').commands.create(commandData);
});

And that's it! As soon as your client is ready, it will register the echo command.

Handling Commands

Now let's implement a simple handler for it:

client.on('interaction', interaction => {
  // If the interaction isn't a slash command, return
  if (!interaction.isCommand()) return;

  // Check if it is the correct command
  if (interaction.commandName === 'echo') {
    // Get the input of the user
    const input = interaction.options[0].value;
    // Reply to the command
    interaction.reply(input);
  }
});

The interaction event will get emitted every time the client receives an interaction. Only our own slash commands trigger this event, so there is no need to implement a check for commands that belong to other bots.