v8.0.0-alpha300.11Published 3 months ago


Source code of released version | Source code of development version

Babel is a parser and transpiler for ECMAScript 2015 syntax and beyond, which enables some upcoming JavaScript syntax features to be used in today's browsers and runtimes.

Meteor's Babel support consists of the following core packages:

  • babel-compiler - Exposes the Babel API on the symbol Babel. For example, Babel.compile(source, options).

  • babel-runtime - Meteor versions of the external helpers used by Babel-generated code. Meteor's core packages must run on IE 8 without polyfills, so these helpers cannot assume the existence of Object.defineProperty, Object.freeze, and so on.

Babel API

The babel-compiler package exports the Babel symbol, which exposes functionality provided by the @meteorjs/babel NPM package, which is in turn implmented using the babel-core NPM package. Note that you can only use the babel-compiler package on the server.


1var babelOptions = Babel.getDefaultOptions();
3// Modify the default options, if necessary:
4babelOptions.whitelist = [
5  "es6.blockScoping", // For `let`
6  "es6.arrowFunctions" // For `=>`
9var result = Babel.compile(
10  "let square = (x) => x*x;",
11  babelOptions
14// result.code will be something like
15// "var square = function (x) {\n  return x * x;\n};"

Use Babel.compile(source) to transpile code using a set of default options that work well for Meteor code.

.babelrc configuration files

Like other Babel-compiled projects, a Meteor project that uses the ecmascript package can specify custom Babel plugins and presets (which are just groups of plugins) in JSON files named .babelrc.

For example, to enable the Babel transform that supports class properties, you should

  1. run meteor npm install --save-dev babel-plugin-transform-class-properties
  2. put the following in a .babelrc file in the root of your project:
2  "plugins": ["transform-class-properties"]

If you want to include all Stage 1 transforms (including the class properties plugin), you could use a preset instead:

meteor npm install --save-dev babel-preset-stage-1

and then (in your .babelrc file):

2  "presets": ["stage-1"]

Note that you should never need to include the es2015 or react transforms, as that functionality is already provided by the default babel-preset-meteor preset.

Any plugins and transforms that you list in your .babelrc file will be included after babel-preset-meteor.

To be considered by the babel-compiler package, .babelrc files must be contained within your root application directory.