Output an ES module using webpack
JavascriptWebpackEcmascript 6Webpack 2Es6 ModulesJavascript Problem Overview
With Rollup I can output an ES module by simply setting the format
option to 'es'
. How can I do the same with webpack? If it's not possible now, does webpack have any plans to add it?
The only thing I've found in the documentation for output.libraryTarget
that mentions ES modules is this:
> libraryTarget: "commonjs-module"
- Expose it using the module.exports
object (output.library
is ignored), __esModule
is defined (it's threaded as ES2015 Module in interop mode)
However, it's rather unclear to me. Is it the same as libraryTarget: "commonjs2"
with the only difference that __esModule
is defined? What is "interop mode"?
Javascript Solutions
Solution 1 - Javascript
Firstly I would like to state the difference between the commonJS
and commonJS2
CommonJS
doesn't support the use of module.exports = function() {}
which is used by node.js
and many other commonJS
implementations.
Webpack2
employs the concept of the bundling the library code and for the widespread use of it and to make it compatible of working in different environments we use the --libraryTarget option
Now the part here will answer both your questions
The possible library options supported in webpack2
are
libraryTarget: "umd", // enum
libraryTarget: "umd-module", // ES2015 module wrapped in UMD
libraryTarget: "commonjs-module", // ES2015 module wrapped in CommonJS
libraryTarget: "commonjs2", // exported with module.exports
libraryTarget: "commonjs", // exported as properties to exports
libraryTarget: "amd", // defined with AMD defined method
libraryTarget: "this", // property set on this
libraryTarget: "var", // variable defined in root scope
Interlop has the following meaning
>In order to encourage the use of CommonJS and ES6 modules, when exporting a default export
with no other exports
module.exports
will be set in addition to exports["default"]
as shown in the following example
export default test;
exports["default"] = test;
module.exports = exports["default"];
So basically it means that the commonJS-module
can be used by exposing it as module.exports
by using the interloping
with ES2015 module wrapped in commonJS
More info about the interloping
can be found in this blogpost and the stackoverflow link to it.
The basic idea is in ES6
runtime export and import properties cannot be changed but in commonJS
this works fine as the require changes happen at runtime therefore it has ES2015 is interloped with the commonJS.
Update
Webpack 2 gives the option of creating the library which can be bundled and included.
If you want your module to be used in different environments then you can bundle it as a library by adding the library options and output it to your specific environment. Procedure mentioned in the docs.
Another simple example on how to use commonjs-module
Important point to note here is babel
adds exports.__esModule = true
to each es6 module
and on importing it calls for the _interopRequire
to check that property.
__esModule = true
need to be set only on library export. It need to be set on the exports
of the entry module. Inner modules don't need __esModule
, it's just a babel hack.
As mentioned in the docs
>__esModule
is defined (it's threaded as ES2015 Module in interop mode)
Usage as mentioned in the test cases
export * from "./a";
export default "default-value";
export var b = "b";
import d from "library";
import { a, b } from "library";
Solution 2 - Javascript
Webpack2 does not have relevant libraryTarget yet, it does not output ES6 bundles. From the other side If you bundle your library in CommonJS bundlers wont be able to run Tree Shaking, not being able to eliminate unused modules. That's due to ES modules are still developing, so nobody ships ES bundles to browser, while webpack used primarily to create browser enabled bundles.
From the other side if you are publishing library you can provide both CommonJS (umd) and ES targets, thanks to "module" key in package. json. Actually you do not need webpack to publish ES target, all you need to do is to run babel on every file to get it to es2015 standart, for example if you are using react you can run babel with just "react" preset. If your source is already ES 2015 without extra features you can point module straight to your src/index.js:
//package.json
...
"module": "src/index.js"
"main": "dist/your/library/bundle.js
...
I found it convenient to use babel to handle export v from 'mod'
instructions in my main index.js, so I have 1 module file exporting all my modules. That's achieved with babel-plugin-transform-export-extensions (also included in stage-1 preset).
I spot this approach from react-bootstrap library, you can see scripts in their github (they are webpack1). I have improved their scripts a little in my react-sigma repo, feel free to copy following files which will do what you need:
config/babel.config.js
scripts/buildBabel.js
scripts/es/build.js
scripts/build.js // this is command line controller, if you need just ES you don't need it
Also look at lib target (scripts/lib/build.js and .babelrc), I provide lib transpiled modules so library users can include only modules they need even without ES explicitly specifying require("react-sigma/lib/Sigma/"), especially useful if your lib is heavy and modular!
Solution 3 - Javascript
What is the best non-hacky solution?
Apparently, this feature is coming in Webpack 5
Update: Webpack 5 was released, but still doesn't support this behavior.
Update 2: See this comment which makes it seem like it is coming soon.
Update 3: The newest version of Webpack should support this use case ! (Although it seems the implementation is still a bit buggy.
If you need to use older versions of Webpack you can use the steps described in this blog post to use a plugin which can allow for outputting an ES module.
When using the blog post above (the post's example code is a little off) we first run
npm i -D @purtuga/esm-webpack-plugin
We can then set webpack.config.js
to the following:
const EsmWebpackPlugin = require("@purtuga/esm-webpack-plugin");
module.exports = {
mode: "development",
entry: "index.js",
output: {
filename: "consoleUtils.js",
library: "LIB",
libraryTarget: "var"
},
//...
plugins: [
new EsmWebpackPlugin()
]
}
Then we can just use the file like normal.
import func from "./bundle.js";
func();
See also this issue which is tracking this feature.
Solution 4 - Javascript
If you don't mind adding an additional file to your package , you can use this workaround as a solution that allows you to distribute/import Webpack bundles as ES6 modules:
Configuration
webpack.config.js
output: {
path: path.resolve('./dist'),
filename: 'bundle.js',
library: '__MODULE_DEFAULT_EXPORT__',
libraryTarget: 'window',
libraryExport: 'default'
},
./dist/index.js (we need to create this file)
import './bundle.js'
const __MODULE_DEFAULT_EXPORT__ = window.__MODULE_DEFAULT_EXPORT__
delete window.__MODULE_DEFAULT_EXPORT__
export default __MODULE_DEFAULT_EXPORT__
package.json (important if you are going to distribute your module)
"main": "dist/index.js",
How it works :
- Webpack outputs the bundle to
/dist/bundle.js
, using thewindow
aslibraryTarget
method. According to my configuration, this makes the package default export available aswindow.__MODULE_DEFAULT_EXPORT__
, as soon as it's imported. - We create a custom "loader" :
./dist/index.js
, which imports the/dist/bundle.js
, picks thewindow.__MODULE_DEFAULT_EXPORT__
, deletes it from thewindow
object (cleanup), assigns it to a local variable, and exports it as a ES6 export. - We configure our
package.json
to point to our "loader" :./dist/index.js
- We can now perform a regular
import MyDefaultExportName from './dist/index.js'
> Note : This solution -as it is exposed here- is far from being perfect, and has some downsides and limitations. Although there is space for improvements :)