Send a message with Discord.js

The top answer is outdated

New way is:

const channel = await client.channels.fetch(<id>);

await channel.send('hi')

To add a little context on getting the channel Id; The list of all the channels is stored in the client.channels property.

A simple console.log(client.channels) will reveal an array of all the channels on that server.

You have an error in your .send() line. The current code that you have was used in an earlier version of the discord.js library, and the method to achieve this has been changed.

If you have a message object, such as in a message event handler, you can send a message to the channel of the message object like so:

message.channel.send("My Message");

An example of that from a message event handler:

client.on("message", function(message) {
  message.channel.send("My Message");
});

---

You can also send a message to a specific channel, which you can do by first getting the channel using its ID, then sending a message to it:

(using async/await)

const channel = await client.channels.fetch(channelID);
channel.send("My Message");

(using Promise callbacks)

client.channels.fetch(channelID).then(channel => {
  channel.send("My Message");
});

---

Works as of Discord.js version 12

The send code has been changed again. Both the items in the question as well as in the answers are all outdated. For version 12, below will be the right code. The details about this code are available in this link.

To send a message to specific channel

const channel = <client>.channels.cache.get('<id>');
channel.send('<content>');

To send a message to a specific user in DM

const user = <client>.users.cache.get('<id>');
user.send('<content>');

If you want to DM a user, please note that the bot and the user should have at least one server in common.

Hope this answer helps people who come here after version 12.

There are four ways you could approach what you are trying to achieve, you can use message.reply("Pong") which mentions the user or use message.channel.send("Pong") which will not mention the user, additionally in discord.js you have the option to send embeds which you do through:

client.on("message", () => {
    var message = new Discord.MessageEmbed()
      .setDescription("Pong") // sets the body of it
      .setColor("somecolor")
      .setThumbnail("./image");
      .setAuthor("Random Person")
      .setTitle("This is an embed")
    msg.channel.send(message) // without mention
    msg.reply(message) // with mention
})

There is also the option to dm the user which can be achieved by:

client.on("message", (msg) => {
msg.author.send("This is a dm")

})

See the official documentation.