Commands

Gator is a CLI application, and like many CLI applications, it has a set of valid commands. For example:

gator login - sets the current user in the config
gator register - adds a new user to the database
gator users - lists all the users in the database
etc.

We'll be hand-rolling our CLI rather than using a framework like commander.js. This will give us a better understanding of how CLI applications work.

Assignment

Let's start with a simple login command. For now, all it will do is set the current user in the config file. Usage:

npm run start login <username>

We want to add many commands to our CLI, so let's build a flexible system that will allow us to register new commands easily.

Create a CommandHandler type. A CommandHandler takes a cmdName string and variadic string arguments, For example, in the case of the login command handler, the name would be "login" and the handler will expect the first argument after to be one string, the username. CLI commands usually either modify state or write to stdout, so its return type will be void.

type CommandHandler = (cmdName: string, ...args: string[]) => void;

Variadic parameters (...args) are treated as optional in TypeScript, meaning we can omit them later for commands that don't need them.
A function satisfies a type as long as it includes all required parameters and matches the return type.

Create a login handler function: function handlerLogin(cmdName: string, ...args: string[]).
- If the command's args array is empty, throw an error; the login handler expects a single argument, the username.
- Use the setUser function to set the user to the given username.
- Print a message to the terminal that the user has been set.
Create a CommandsRegistry type. The command registry will just be an object with keys of command names and values of CommandHandler functions. This will hold all the commands the CLI can handle.
Create the following functions:

function registerCommand(registry: CommandsRegistry, cmdName: string, handler: CommandHandler) - This function registers a new handler function for a command name.
function runCommand(registry: CommandsRegistry, cmdName: string, ...args: string[]) - This function runs a given command with the provided state if it exists.

In the main function, remove the manual update of the config file. Whenever we need the config in our handlers, we'll just read from it again.
Create a new CommandsRegistry object.
Register a handler function for the login command.
Use process.argv to get the command-line arguments passed in by the user. Have a look at the arguments and remove any you don't need.

After slicing the arguments, if there isn't at least one argument, print an error message to the terminal and exit with code 1.

Split the command-line arguments into the command name and the arguments array. Use the runCommand function to run the given command and print any errors that are thrown.
Test your CLI. Here are a few cases to consider:
- npm run start - Exit with code 1 and print an error that not enough arguments were provided.
- npm run start login - Exit with code 1 and print an error that a username is required.
- npm run start login alice - should set the current user in the config file to "alice" (and not overwrite the DB string)

Run and submit the CLI tests from the root of your project.

Tip

You can use the Record utility type to create a map of command names to handler functions.