A basic node.js REPL written in ES6.

cli.js

import readline from 'readline';
import util			from 'util';
import chalk 		from 'chalk';


let terminal = readline.createInterface(process.stdin, process.stdout);


export default function cli(commands){
	return (new CLI(commands)).repl();
}

class CLI {

	constructor(commands){
		this.commands = commands || new Map();
	}

	/**
	 * Creates a REPL interface for the user to interact with.
	 */
	repl(){

		terminal.setPrompt('cli> ');
		terminal.prompt();

		terminal.on('line', (line) => {

			let normalized = line.trim();

			let command = this.findCommand(normalized);
			if (!command){
				this.showUsage(normalized);
			}
			else{
				command.exec(normalized.split(' '));
			}

		  terminal.prompt();
		});

		terminal.on('close', () => {
		  process.exit(0);
		});
	}

	/**
	 * Attempts to find the specified command and returns null if it
	 * cannot be found.
	 * @param   {String} command The command to find.
	 * @returns {Object} The command or null.
	 */
	findCommand(command){

		let normalized = command.trim().toLowerCase();

		if (this.commands.has(normalized)){
			return this.commands.get(normalized);
		}

		return null;
	}

	/**
	 * Shows the usage of the application based on the registered
	 * commands found in the CLI registry.
	 */
	showUsage(invalidCommand){

		terminal.write(util.format('%s\n\n', chalk.blue('== THE CLI =='));

		for (let i =0; i < this.commands.length; i++){
			terminal.write(util.format('--%s ', chalk.gray(this.commands[i].name) ||''));
		}

		terminal.write('\n');
	}
}