extracted ESNext transpilation into separate plugin

this resolves the dependencies-management issue (cf. previous commit;
69a2bc2)

opting into features like this is pretty much why we designed extensible
configuration the way we did

creating a separate repository for this plugin didn't seem warranted and
would have complicated testing

lib/config/mapping.js

@@ -1,6 +1,5 @@
 "use strict";
 
-let generateTranspiler = require("./babel");
 let { Plugin } = require("./plugin");
 let { warn, raise, ValidationError } = require("../util");
 
@@ -38,13 +37,6 @@ let NAMELESS_FORMATS = new Set(["es", "amd", "cjs"]); // NB: Rollup identifiers
 // `extensions` (optional; depends on `resolve`) is a list of additional file
 // extensions for implicit module resolution (e.g. `[".wasm"]`)
 //
-// `esnext` (boolean or object) activates ESNext transpilation if truthy
-//  * `esnext.browsers` (optional) is an array of Browserslist queries (e.g.
-//    `["> 1%", "last 2 versions"]`) to determine which features are transpiled
-//  * `esnext.exclude` is a list of modules for which to skip transpilation
-//    (e.g. `exclude: ["jquery"]`, perhaps due to a distribution already
-//    optimized for ES5)
-//
 // `commonJS` (boolean or glob) activates support for importing CommonJS modules
 // limited to `node_modules` directory unless a custom glob pattern is specified
 //
@@ -59,7 +51,6 @@ exports.generateConfig = function generateConfig(entryPoint, {
 	sourcemaps,
 	resolve,
 	extensions = [],
-	esnext,
 	commonjs,
 	treeshaking,
 	parser
@@ -91,9 +82,6 @@ exports.generateConfig = function generateConfig(entryPoint, {
 			include: commonjs === true ? defaultGlob : commonjs
 		}));
 	}
-	if(esnext) {
-		plugins.push(generateTranspiler(esnext));
-	}
 
 	format = MODULE_FORMATS[format.toLowerCase()] ||
 			raise(`unrecognized module format: \`${format}\``, ValidationError,

lib/config/plugin.js

@@ -3,21 +3,28 @@
 let { loadExtension } = require("../util");
 
 exports.Plugin = class Plugin {
-	constructor(id, ref, config = null) {
+	constructor(id, ref, config = null, supplier = null) {
 		this.id = id;
 		this.ref = ref;
 		this.config = config;
+		if(supplier) {
+			this.supplier = supplier;
+		}
 	}
 
 	init() {
 		let { ref, config } = this;
 		// lazy loading
 		if(ref.substr) {
-			ref = loadExtension(ref, `plugin \`${this.id}\``);
+			ref = this._load(ref);
 		} else if(ref.module && ref.import) {
-			ref = loadExtension(ref.module, `plugin \`${this.id}\``)[ref.import];
+			ref = this._load(ref.module)[ref.import];
 		}
 
 		return config === null ? ref : ref(config);
 	}
+
+	_load(pkg) {
+		return loadExtension(pkg, `plugin \`${this.id}\``, this.supplier);
+	}
 };

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "beatdown",
-	"version": "1.0.1",
+	"version": "1.0.2",
 	"description": "simplified JavaScript module bundling",
 	"author": "FND",
 	"license": "Apache-2.0",
@@ -22,14 +22,12 @@
 		"node": ">=10"
 	},
 	"dependencies": {
-		"rollup": "^2.18.1"
+		"rollup": "^2.19.0"
 	},
 	"devDependencies": {
-		"@babel/core": "^7.10.3",
-		"@babel/preset-env": "^7.10.3",
-		"@rollup/plugin-babel": "^5.0.4",
 		"@rollup/plugin-commonjs": "^13.0.0",
 		"@rollup/plugin-node-resolve": "^8.1.0",
+		"beatdown-esnext": "file:./pkg/esnext",
 		"eslint-config-fnd": "^1.8.0",
 		"mocha": "^8.0.1",
 		"npm-run-all": "^4.1.5",

lib/config/babel.js → pkg/esnext/index.js

@@ -1,20 +1,24 @@
 "use strict";
 
-let { Plugin } = require("./plugin");
-let { report } = require("../util");
+let { Plugin } = require("beatdown/lib/config/plugin");
+let { report } = require("beatdown/lib/util");
 
+let PRESET_ENV = "@babel/preset-env";
+
+//* `browsers` (optional) is an array of Browserslist queries (e.g.
+//  `["> 1%", "last 2 versions"]`) to determine which features are transpiled
+//* `exclude` (optional) is a list of modules for which to skip transpilation
+//  (e.g. `exclude: ["jquery"]`, perhaps due to the respective library's
+//  distribution already being optimized for ES5)
 module.exports = function generateTranspiler({ browsers, exclude }) {
 	if(browsers && browsers.length) {
 		report("transpiling JavaScript for", browsers.join(", "));
 	}
-	return new Plugin("babel", {
-		// FIXME: @babel/core and @babel/preset-env are additional dependencies
-		module: "@rollup/plugin-babel",
-		import: "default"
-	}, {
+
+	let config = {
 		babelHelpers: "bundled",
 		presets: [
-			["@babel/preset-env", {
+			[PRESET_ENV, {
 				modules: false,
 				...(browsers && {
 					targets: { browsers }
@@ -30,5 +34,14 @@ module.exports = function generateTranspiler({ browsers, exclude }) {
 				return /^\.{0,2}\//.test(pkg) ? pkg : `node_modules/${pkg}/**`;
 			})
 		})
-	});
+	};
+
+	let plugin = new Plugin("babel", {
+		module: "@rollup/plugin-babel",
+		import: "default"
+	}, config, "beatdown-babel");
+	// ensure controlled error handling for implicit dependency
+	plugin._load(PRESET_ENV);
+
+	return plugin;
 };

pkg/esnext/package.json

@@ -0,0 +1,22 @@
+{
+	"name": "beatdown-esnext",
+	"version": "1.0.0",
+	"description": "simplified JavaScript module bundling",
+	"author": "FND",
+	"license": "Apache-2.0",
+	"homepage": "https://www.faucet-pipeline.org",
+	"repository": {
+		"type": "git",
+		"url": "https://github.com/faucet-pipeline/beatdown.git"
+	},
+	"bugs": {
+		"url": "https://github.com/faucet-pipeline/beatdown/issues"
+	},
+	"main": "index.js",
+	"dependencies": {
+		"@babel/core": "^7.10.4",
+		"@babel/preset-env": "^7.10.4",
+		"@rollup/plugin-babel": "^5.0.4",
+		"beatdown": "1.0.2"
+	}
+}

test/test_transpiling.js

@@ -1,6 +1,7 @@
 /* global suite, test */
 "use strict";
 
+let generateTranspiler = require("../pkg/esnext");
 let { Bundle } = require("../lib/bundle");
 let { Config } = require("../lib/config");
 let { FIXTURES_DIR, fixturePath } = require("./util");
@@ -17,7 +18,7 @@ let warn = (...msg) => console.error(...msg);let lipsum = lang => \`[$\{lang}] l
 	`.trim());
 	/* eslint-enable max-len */
 
-	bundle = makeBundle("./index.esnext.js", { esnext: true });
+	bundle = makeBundle("./index.esnext.js", {});
 	code = await bundle.compile();
 	assertSame(code, `
 var warn = function warn() {
@@ -34,9 +35,7 @@ var warn = function warn() {
 
 test("Browserslist support", async () => {
 	let bundle = makeBundle("./index.esnext.js", {
-		esnext: {
-			browsers: ["current Node"]
-		}
+		browsers: ["current Node"]
 	});
 	let code = await bundle.compile();
 	/* eslint-disable max-len */
@@ -50,9 +49,7 @@ test("selectively skipping transpilation", async () => {
 	let cwd = process.cwd();
 	process.chdir(FIXTURES_DIR); // FIXME: smell
 	let bundle = makeBundle("./index.esnext.js", {
-		esnext: {
-			exclude: ["mylib"]
-		}
+		exclude: ["mylib"]
 	});
 	let code = await bundle.compile();
 	process.chdir(cwd);
@@ -69,11 +66,14 @@ var warn = function warn() {
 	/* eslint-enable max-len */
 });
 
-function makeBundle(entryPoint, options) {
+function makeBundle(entryPoint, esnext) {
 	let config = new Config(fixturePath(entryPoint), {
 		compact: true,
-		resolve: true,
-		...options
+		resolve: true
 	});
+	if(esnext) {
+		let plugin = generateTranspiler(esnext);
+		config.addPlugin(plugin);
+	}
 	return new Bundle(config);
 }