# Making an Eval command

# What is eval exactly?

In JavaScript (and node), eval() is a function that evaluates any string as javascript code and actually executes it. Meaning, if I eval(2+2), eval will return 4. If I eval client.guilds.cache.size, it'll return however many guilds the bot is currently on. And if I eval client.guilds.cache.map(g=>g.name).join('\n') then it will return a list of guild names separated by a line return. Cool, right?

# But eval is dangerous

I'll say this in a way that's probably dead simple to understand: Giving someone access to eval() is literally like sitting them at your computer, with full admin access, and then stepping out of the room. eval() in browser javascript is trivial and not dangerous - you're running in your own browser, anything you fuck up is going to be on your own PC, not the web servers.

But eval() in node is really, really dangerous and powerful. Because it can run anything you run as a bot, and it can also run code you're not expecting to run, if someone else has access to it. Node.js has access to your hard drive. The whole thing. Every bit of it. To understand what this means, look at the following command: rm -rf / --no-preserve-root . Do you know what this command does? It deletes your entire server's hard drives. I mean, it only works on Linux, but most VPS systems and most hosting providers are on UNIX-based systems.

Another thing that's on your hard drive is passwords. Have a config file with your database password? I can get that. config.json with your token and other API keys in it? I can grab that easy. Hell I can just grab your token straight from the client object if I know how to.

# Securing your eval

So first, we need to understand the #1 rule when using eval commands:

I don't care if it's a server owner, someone you've been talking to for months, you cannot trust anyone with eval. There's only one exception to this rule: Someone you know in real life that you can punch in the face when they actually destroy half your server or mistakenly ban everyone in every server your bot is in. Eval bypasses any command-based permission you might have, it bypasses all security checks. Eval is all powerful.

So how do you secure it? Simple: only allow use from your own user ID. So for example my user ID is 139412744439988224 so I check whether the message author's ID is mine, which we added into our config at the start:

  // The rest of the config
  "ownerID": "139412744439988224"

In the code for the bot:

// If the message author's ID does not equal
// our ownerID, get outta there!
if (message.author.id !== config.ownerID) return;

It's as simple as that to protect the command directly inside of your condition or file or whatever. Of course, if you have some sort of command handler there's most likely a way to restrict to an ID too. This isn't specific to discord.js: there's always a way to do this. If there isn't (if a command handler won't let you restrict by ID), then find a new library!.

# Simple Eval Command

So now you've been thoroughly briefed on the dangers of Eval, let's take a look at how to implement a simple eval command.

First though I strongly suggest using the following function (plop it outside of any event handler/functions you have, so it's accessible anywhere). This function prevents the use of actual mentions within the return line by adding a zero-width character between the @ and the first character of the mention - blocking the mention from happening.

// This function cleans up and prepares the
// result of our eval command input for sending
// to the channel
const clean = async (text) => {
// If our input is a promise, await it before continuing
if (text && text.constructor.name == "Promise")
  text = await text;

// If the response isn't a string, `util.inspect()`
// is used to 'stringify' the code in a safe way that
// won't error out on objects with circular references
// (like Collections, for example)
if (typeof text !== "string")
  text = require("util").inspect(text, { depth: 1 });

// Replace symbols with character code alternatives
text = text
  .replace(/`/g, "`" + String.fromCharCode(8203))
  .replace(/@/g, "@" + String.fromCharCode(8203));

// Send off the cleaned up result
return text;

Alright, So let's get down to the brass tax: The actual eval command. Here it is in all its glory, assuming you've followed this guide all along:

// This command goes inside of our message event handler
client.on("messageCreate", async (message) => {

  // Get our input arguments
  const args = message.content.split(" ").slice(1);

  // The actual eval command
  if (message.content.startsWith(`${config.prefix}eval`)) {

    // If the message author's ID does not equal
    // our ownerID, get outta there!
    if (message.author.id !== config.ownerID)

    // In case something fails, we to catch errors
    // in a try/catch block
    try {
      // Evaluate (execute) our input
      const evaled = eval(args.join(" "));

      // Put our eval result through the function
      // we defined above
      const cleaned = await clean(evaled);

      // Reply in the channel with our result
    } catch (err) {
      // Reply in the channel with our error
      message.channel.send(`\`ERROR\` \`\`\`xl\n${cleaned}\n\`\`\``);

    // End of our command

  // End of our message event handler

That's it. That's the command. Note a couple of things though:

  • Your IDE or editor may scream at the eval(code) for being unsafe. See the rest of this page for WHY it says that. Can't say you haven't been warned!
  • If the response is more than 2000 characters this will error when it tries to send the results. This is due to the character limit of messages sent to channels. You will need to handle splitting this up and sending the larger results if you want to be able to send larger results.

Adjust our clean() function to receive the client as an input like so.

const clean = async (client, text) => { 
  // The rest of the code

Be sure to supply the client as an input when we call the function later in the code.

    // ...

    try {
      // Put our eval result through the function we defined above
      const cleaned = await clean(client, evaled);

    // ...
const clean = async (client, text) => { 
  // ...

  // You will need to place this inside the clean
  // function, before the result is returned.
  text = text.replaceAll(client.token, "[REDACTED]");

  // ...

# Final Reminder

Hopefully the warnings were clear enough to help you understand the dangers... but the idea of eval is still attractive enough that you'll use it for yourself anyway!