Enable yargs strict, add 'start' alias for serve command

Summary:
Fixes command validation in Metro CLI.

- Enables `yargs.strict()`, which prevents Metro from exiting silently if called with a nonexistent subcommand or option.
- Adds `start` as an alias for `serve` command — matching RN CLI.
- Simplifies `yargs.command` calls in index module by aligning `desc` key name to match yargs `ModuleObject` type.

Changelog:

**[Breaking]** Enable strict CLI validation when passed an invalid subcommand or argument
**[Feature]** Add `start` alias for `serve` CLI command

Reviewed By: motiz88

Differential Revision: D37922367

fbshipit-source-id: 54ba4f714d82ad3d825236d2982c6f4be02b07e5

Author: huntie

packages/metro/src/cli-utils.js

@@ -27,7 +27,7 @@ exports.watchFile = async function (
 
 exports.makeAsyncCommand =
   (
-    command: (argv: YargArguments) => Promise<mixed>,
+    command: (argv: YargArguments) => Promise<void>,
   ): ((argv: YargArguments) => void) =>
   (argv: YargArguments) => {
     Promise.resolve(command(argv)).catch(error => {

packages/metro/src/commands/build.js

@@ -13,6 +13,7 @@
 import type {RunBuildOptions} from '../index';
 import type {YargArguments} from 'metro-config/src/configTypes.flow';
 import typeof Yargs from 'yargs';
+import type {ModuleObject} from 'yargs';
 
 const {makeAsyncCommand} = require('../cli-utils');
 const TerminalReporter = require('../lib/TerminalReporter');
@@ -22,16 +23,12 @@ const {Terminal} = require('metro-core');
 const term = new Terminal(process.stdout);
 const updateReporter = new TerminalReporter(term);
 
-module.exports = (): ({
-  builder: (yargs: Yargs) => void,
-  command: string,
-  description: string,
-  handler: (argv: YargArguments) => void,
-}) => ({
+module.exports = (): {
+  ...ModuleObject,
+  handler: Function,
+} => ({
   command: 'build <entry>',
-
-  description:
-    'Generates a JavaScript bundle containing the specified entrypoint and its descendants',
+  desc: 'Generates a JavaScript bundle containing the specified entrypoint and its descendants',
 
   builder: (yargs: Yargs): void => {
     yargs.option('project-roots', {

packages/metro/src/commands/dependencies.js

@@ -10,6 +10,7 @@
 'use strict';
 
 import typeof Yargs from 'yargs';
+import type {ModuleObject} from 'yargs';
 
 const {makeAsyncCommand} = require('../cli-utils');
 const Server = require('../Server');
@@ -67,9 +68,9 @@ async function dependencies(args: any, config: any) {
     : Promise.resolve();
 }
 
-module.exports = () => ({
+module.exports = (): ModuleObject => ({
   command: 'get-dependencies',
-  description: 'List dependencies',
+  desc: 'List dependencies',
   builder: (yargs: Yargs) => {
     yargs.option('entry-file', {
       type: 'string',

packages/metro/src/commands/serve.js

@@ -13,20 +13,19 @@
 import type {RunServerOptions} from '../index';
 import type {YargArguments} from 'metro-config/src/configTypes.flow';
 import typeof Yargs from 'yargs';
+import type {ModuleObject} from 'yargs';
 
 const {makeAsyncCommand, watchFile} = require('../cli-utils');
 const {loadConfig, resolveConfig} = require('metro-config');
 const {promisify} = require('util');
 
-module.exports = (): ({
-  builder: (yargs: Yargs) => void,
-  command: $TEMPORARY$string<'serve'>,
-  description: string,
-  handler: (argv: YargArguments) => void,
-}) => ({
+module.exports = (): {
+  ...ModuleObject,
+  handler: Function,
+} => ({
   command: 'serve',
-
-  description: 'Starts Metro on the given port, building bundles on the fly',
+  aliases: ['start'],
+  desc: 'Starts Metro on the given port, building bundles on the fly',
 
   builder: (yargs: Yargs): void => {
     yargs.option('project-roots', {

packages/metro/src/index.flow.js

@@ -453,18 +453,18 @@ exports.attachMetroCli = function (
   // $FlowFixMe[prop-missing]
   = {},
 ): Yargs {
+  yargs.strict();
+
   if (build) {
-    const {command, description, builder, handler} = makeBuildCommand();
-    yargs.command(command, description, builder, handler);
+    yargs.command(makeBuildCommand());
   }
   if (serve) {
-    const {command, description, builder, handler} = makeServeCommand();
-    yargs.command(command, description, builder, handler);
+    yargs.command(makeServeCommand());
   }
   if (dependencies) {
-    const {command, description, builder, handler} = makeDependenciesCommand();
-    yargs.command(command, description, builder, handler);
+    yargs.command(makeDependenciesCommand());
   }
+
   return yargs;
 };