What’s new in webpack 2
Work in progress… This reflects stuff until 2.0.5-beta
Major changes
ES6 Modules
webpack 2 brings native support ES6 Modules. This means webpack now understands import
and export
without them being transformed to CommonJS:
import { currentPage, readPage } from "./book";
currentPage === 0;
readPage();
currentPage === 1;
// book.js
export var currentPage = 0;
export function readPage() {
currentPage++;
}
export default "This is a book";
Code Splitting with ES6
The ES6 Loader spec defines System.import
as method to load ES6 Modules dynamically on runtime.
Webpack threads System.import
as splitpoint and puts the requested module in a separate chunk.
System.import
takes the module name as argument and returns a Promise.
function onClick() {
System.import("./module").then(module => {
module.default;
}).catch(err => {
console.log("Chunk loading failed");
});
}
Good news: Failure to load a chunk can be handled now.
Dynamic expressions
It’s possible to pass an partial expression to System.import
. This is handled similar to expressions in CommonJS (webpack creates a context with all possible files).
System.import
creates a separate chunk for each possible module.
function route(path, query) {
return System.import("./routes/" + path + "/route")
.then(route => new route.Route(query));
}
// This creates a separate chunk for each possible route
Mixing ES6 with AMD and CommonJS
As for AMD and CommonJS you can freely mix all three module types (even within the same file). Webpack behaves similar to babel in this case:
// CommonJS consuming ES6 Module
var book = require("./book");
book.currentPage;
book.readPage();
book.default === "This is a book";
// ES6 Module consuming CommonJS
import fs from "fs"; // module.exports map to default
import { readFileSync } from "fs"; // named exports are read from returned object+
typeof fs.readFileSync === "function";
typeof readFileSync === "function";
babel and webpack
The es2015
babel preset transforms ES6 Modules to CommonJS by default. To use webpack to process ES6 Modules you should use the es2015-webpack
preset instead.
ES6 specific optimizations
The static nature of ES6 Modules allows some new kind of optimizations. In example in many cases it’s possible to detect which exports are used and which aren’t used.
In cases in which webpack can say for sure that an export isn’t used it omits the statement which exposes the export to other modules. Later the minimizer may flag the declaration as unused and omits it.
In the following cases it’s possible to detect usage:
- named import
- default import
- reexport
In the following cases it’s not possible to detect usage:
import * as ...
- CommonJS or AMD consuming ES6 module
System.import
Configuration
In the past enviroment variables are often used to handle different enviroments in the configuration file. Webpack 2 brings a new way to pass options to the configuration.
The configuration file can export a function which returns the configuration. The function is called by the CLI and the value passed via --env
is passed to the configuration function.
You can pass a string (--env dev
=> "dev"
) or a complex options object (--env.minimize --env.server localhost
=> {minimize: true, server: "localhost"}
). I would recommend using an object, because it’s more extendable, but it’s up to you.
Example
// webpack.config.babel.js
exports default function(options) {
return {
// ...
devtool: options.dev ? "cheap-module-eval-source-map" : "hidden-source-map"
};
}
Resolving options
There was a major refactoring in the resolver (github.com/webpack/enh…). This means the resolving option were changed too. Mostly simplification and changes that make it more unlikly to configurate it incorrectly.
The new options are:
{
modules: [path.resolve(__dirname, "app"), "node_modules"]
// (was split into `root`, `modulesDirectories` and `fallback` in the old options)
// In which folders the resolver look for modules
// relative paths are looked up in every parent folder (like node_modules)
// absolute paths are looked up directly
// the order is respected
descriptionFiles: ["package.json", "bower.json"],
// These JSON files are read in directories
mainFields: ["main", "browser"],
// These fields in the description files are looked up when trying to resolve the package directory
mainFiles: ["index"]
// These files are tried when trying to resolve a directory
aliasFields: ["browser"],
// These fields in the description files offer aliasing in this package
// The content of these field is an object where requests to a key are mapped to the corresponding value
extensions: [".js", ".json"],
// These extensions are tried when resolving a file
enforceExtension: false,
// If false it will also try to use no extension from above
moduleExtensions: ["-loader"],
// These extensions are tried when resolving a module
enforceModuleExtension: false,
// If false it's also try to use no module extension from above
alias: {
jquery: path.resolve(__dirname, "vendor/jquery-2.0.0.js")
}
// These aliasing is used when trying to resolve a module
}
Minor breaking changes
Promise
polyfill
The chunk loading stuff now relies on Promise
being available. This means you need to provide a Promise
polyfill for older browsers.
The ES6 spec uses promises and I don’t want to include a Promise polyfill in every bundle. So it’s up the application developer to provide the polyfill if needed.
Loaders configuration
The loaders in the configuration now match to the resourcePath
instead of the resource
. This means the query string is no longer included for matching.
This was an issue with bootstrap which complicates the test
for bootstrap fonts and images from /\.svg$/
to /\.svg($|\?)/
. Now you can use the simple form.
The loader in the configuration now resolves relative to the configuration file (or the context
option in the configuration file if specified). This should fix some issues with npm link
ed modules that are outside of the current package.
Another change allows the following syntax to configurate loaders:
loaders: [
{
test: /\.css$/,
loaders: [
"style-loader",
{ loader: "css-loader", query: { modules: true } },
{
loader: "sass-loader",
query: {
includePaths: [
path.resolve(__dirname, "some-folder")
]
}
}
]
}
]
Plugins
Many plugins now take option objects instead of multiple arguments. This happens because it is easier to extend. They throw an Error when the old argument style is passed.
HMR communication
In webpack 1 the update signal used the Web Messaging API (postMessage
). Webpack 2 uses a standard event emitter to receive the event. This means WebSocket must be inline in the bundle.
webpack-dev-server has inlined mode as default now.
This should allow to use the webpack-dev-server to update code in WebWorkers.