cli(prompt): initial comment for prompt file

bin/cli.js

@@ -41,9 +41,7 @@
 	});
 
 	if (NON_COMPILATION_CMD) {
-		// TODO: prompt if package isn't installed, run installed package otherwise.
-		//return require(`./commands/${NON_COMPILATION_CMD}`)(...process.argv);
-		return;
+		return require('./prompt-command')(NON_COMPILATION_CMD, ...process.argv);
 	}
 
 	const yargs = require("yargs").usage(`webpack-cli ${

bin/prompt-command.js

@@ -1 +1,81 @@
-module.exports = {};
+// based on https://github.com/webpack/webpack/blob/master/bin/webpack.js
+module.exports = function promptForInstallation(command, options) {
+	const cp = require("child_process");
+	return new Promise((resolve, reject) => {
+		const executedCommand = cp.spawn(command, options, {
+			stdio: "inherit",
+			shell: true
+		});
+
+		executedCommand.on("error", error => {
+			reject(error);
+		});
+
+		executedCommand.on("exit", code => {
+			if (code === 0) {
+				resolve(true);
+			} else {
+				reject();
+			}
+		});
+	});
+}
+
+let packageIsInstalled = false;
+try {
+	require.resolve(package);
+	packageIsInstalled = true;
+} catch (err) {
+	packageIsInstalled = false;
+}
+
+if (!packageIsInstalled) {
+	const path = require("path");
+	const fs = require("fs");
+	const readLine = require("readline");
+	const isYarn = fs.existsSync(path.resolve(process.cwd(), "yarn.lock"));
+
+	const packageManager = isYarn ? "yarn" : "npm";
+	const options = ["install", "-D", package];
+
+	if (isYarn) {
+		options[0] = "add";
+	}
+
+	const commandToBeRun = `${packageManager} ${options.join(" ")}`;
+
+	const question = `Would you like to install ${packageIsInstalled}? (That will run ${commandToBeRun}) (yes/NO)`;
+
+	console.error(`The CLI moved into a separate package: @webpack-cli/${package}`);
+	const questionInterface = readLine.createInterface({
+		input: process.stdin,
+		output: process.stdout
+	});
+	questionInterface.question(question, answer => {
+		questionInterface.close();
+		switch (answer.toLowerCase()) {
+			case "y":
+			case "yes":
+			case "1": {
+				runCommand(packageManager, options)
+					.then(result => {
+						return require(`@webpack-cli/${package}`); //eslint-disable-line
+					})
+					.catch(error => {
+						console.error(error);
+						process.exitCode = 1;
+					});
+				break;
+			}
+			default: {
+				console.error(
+					"It needs to be installed alongside webpack to use the CLI"
+				);
+				process.exitCode = 1;
+				break;
+			}
+		}
+	});
+} else {
+	require(package); // eslint-disable-line
+}