Seamless integration between Rollup and Babel.
If you're using Babel to transpile your ES6/7 code and Rollup to generate a standalone bundle, you have a couple of options:
- run the code through Babel first, being careful to exclude the module transformer, or
- run the code through Rollup first, and then pass it to Babel.
Both approaches have disadvantages – in the first case, on top of the additional configuration complexity, you may end up with Babel's helpers (like classCallCheck
) repeated throughout your code (once for each module where the helpers are used). In the second case, transpiling is likely to be slower, because transpiling a large bundle is much more work for Babel than transpiling a set of small files.
Either way, you have to worry about a place to put the intermediate files, and getting sourcemaps to behave becomes a royal pain.
Using Rollup with rollup-plugin-babel makes the process far easier.
babel 7.x
npm install --save-dev rollup-plugin-babel@latest
babel 6.x
npm install --save-dev rollup-plugin-babel@3
import { rollup } from 'rollup';
import babel from 'rollup-plugin-babel';
rollup({
entry: 'main.js',
plugins: [
babel({
exclude: 'node_modules/**'
})
]
}).then(...)
All options are as per the Babel documentation, except the following:
options.externalHelpers
: a boolean value indicating whether to bundle in the Babel helpersoptions.include
andoptions.exclude
: each a minimatch pattern, or array of minimatch patterns, which determine which files are transpiled by Babel (by default, all files are transpiled)options.externalHelpersWhitelist
: an array which gives explicit control over which babelHelper functions are allowed in the bundle (by default, every helper is allowed)options.extensions
: an array of file extensions that Babel should transpile (by default the Babel defaults of .js, .jsx, .es6, .es, .mjs. are used)
Babel will respect .babelrc
files – this is generally the best place to put your configuration.
Ideally, you should only be transforming your source code, rather than running all of your external dependencies through Babel – hence the exclude: 'node_modules/**'
in the example above. If you have a dependency that exposes untranspiled ES6 source code that doesn't run in your target environment, then you may need to break this rule, but it often causes problems with unusual .babelrc
files or mismatched versions of Babel.
We encourage library authors not to distribute code that uses untranspiled ES6 features (other than modules) for this reason. Consumers of your library should not have to transpile your ES6 code, any more than they should have to transpile your CoffeeScript, ClojureScript or TypeScript.
Use babelrc: false
to prevent Babel from using local (i.e. to your external dependencies) .babelrc
files, relying instead on the configuration you pass in.
In some cases Babel uses helpers to avoid repeating chunks of code – for example, if you use the class
keyword, it will use a classCallCheck
function to ensure that the class is instantiated correctly.
By default, those helpers will be inserted at the top of the file being transformed, which can lead to duplication. This rollup plugin automatically deduplicates those helpers, keeping only one copy of each one used in the output bundle. Rollup will combine the helpers in a single block at the top of your bundle. To achieve the same in Babel 6 you must use the external-helpers
plugin.
Alternatively, if you know what you're doing, you can use the transform-runtime
plugin. If you do this, use runtimeHelpers: true
:
rollup.rollup({
...,
plugins: [
babel({ runtimeHelpers: true })
]
}).then(...)
By default externalHelpers
option is set to false
so babel helpers will be included in your bundle.
If you do not wish the babel helpers to be included in your bundle at all (but instead reference the global babelHelpers
object), you may set the externalHelpers
option to true
:
rollup.rollup({
...,
plugins: [
babel({
plugins: ['external-helpers'],
externalHelpers: true
})
]
}).then(...)
This is not needed for Babel 7 - it knows automatically that Rollup understands ES modules & that it shouldn't use any module transform with it. The section below describes what needs to be done for Babel 6.
The env
preset includes the transform-es2015-modules-commonjs plugin, which converts ES6 modules to CommonJS – preventing Rollup from working. Since Babel 6.3 it's possible to deactivate module transformation with "modules": false
. So there is no need to use the old workaround with babel-preset-es2015-rollup
, that will work for Babel <6.13. Rollup will throw an error if this is incorrectly configured.
However, setting modules: false
in your .babelrc
may conflict if you are using babel-register
. To work around this, specify babelrc: false
in your rollup config. This allows Rollup to bypass your .babelrc
file. In order to use the env
preset, you will also need to specify it with modules: false
option:
plugins: [
babel({
babelrc: false,
presets: [['env', { modules: false }]],
}),
];
The following applies to Babel 6 only. If you're using Babel 5, do npm i -D rollup-plugin-babel@1
, as version 2 and above no longer supports Babel 5
npm install --save-dev rollup-plugin-babel@3 babel-preset-env babel-plugin-external-helpers
// .babelrc
{
"presets": [
[
"env",
{
"modules": false
}
]
],
"plugins": [
"external-helpers"
]
}
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