Send a message with Discord.js

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 Both the items in the question as well as in the answers are all outdated.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.

The top answer is outdated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.

Below is the code to dm the user:

(In this case our message is not a response but a new message sent directly to the selected user.)

require('dotenv').config({ path: __dirname + '/.env.local' });

const Discord = require("discord.js");
const client = new Discord.Client();

client.on("ready", () => {
    console.log(client.users.get('ID_OF_USER').send("hello"));
});

client.login(process.env.DISCORD_BOT_TOKEN);

Further documentation:

https://github.com/AnIdiotsGuide/discordjs-bot-guide/blob/master/frequently-asked-questions.md#users-and-members

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.

You can only send a message to a channel

client.on('message', function(message) {
 if (message.content === 'ping') {
  message.channel.send('pong');
 }
});

If you want to DM the user, then you can use the User.send() function

client.on('message', function(message) {
 if (message.content === 'ping') {
  message.author.send('pong');
 }
});

Types of ways to send a message: DM'ing whoever ran the command:

client.on('message', function(message) {
 if (message.content === 'ping') {
  message.author.send('pong');
 }
});

Sends the message in the channel that the command was used in:

client.on('message', function(message) {
 if (message.content === 'ping') {
  message.channel.send('pong');
 }
});

Sends the message in a specific channel:

client.on('message', function(message) {
const channel = client.channels.get("<channel id>")
 if (message.content === 'ping') {
  channel.send("pong")
 }
});

It's message.channel.send("content"); since you're sending a message to the current channel.