# Using argument validators

Sometimes you're going to want an argument to be a certain thing, e.g., check if it's a specific piece of text or check the length. A validate function in your arg can accomplish this.

# Basic validators

What if you have a command where your first argument has to match a specific text? For example: if you wanted your say command to allow a maximum of 200 characters. It's straightforward.

Pull the argument from your say command and add a blank validate function to the arg.:










 






module.exports = class SayCommand extends Command {
	constructor(client) {
		super(client, {
			// ...
			args: [
				{
					key: 'text',
					prompt: 'What text would you like the bot to say?',
					type: 'string',
					validate: text => {},
				},
			],
		});
	}
};

Inside your validate function, check to see if the length is below 201 characters.










 






module.exports = class SayCommand extends Command {
	constructor(client) {
		super(client, {
			// ...
			args: [
				{
					key: 'text',
					prompt: 'What text would you like the bot to say?',
					type: 'string',
					validate: text => text.length < 201,
				},
			],
		});
	}
};

And now you've got a validator that checks if the length is 200!

# oneOf

Another property you can use to validate arguments is the oneOf option. This option forces the argument to be one of the options provided in an array. If you wanted to make an argument that required a "yes" or "no" response, you'd do this:







 
 
 
 






module.exports = class SayCommand extends Command {
	constructor(client) {
		super(client, {
			// ...
			args: [
				{
					key: 'option',
					prompt: 'Yes or No?',
					type: 'string',
					oneOf: ['yes', 'no'],
				},
			],
		});
	}
};

That's it! This will automatically be case-insensitive, so you don't have to worry about that.