6cbc585cf4
Prior to this change, we'd conduct an open-ended traversal on the 'id' of any VariableDeclarator to find a RestElement. The 'id' of a VariableDeclarator can contain an AssignmentPattern (to supply a default value), and if the right-hand side of the AssignmentPattern contained a RestElement, we'd transform it. The problem here is that the right-hand side of an AssignmentPattern can be *any* Expression. If the right-hand side is a function body, we'd traverse the entire function body, and if a RestElement occurred anywhere in that function body, we'd transform it and emit the transformations wherever we began the traversal (at least one scope outside its usage). The fix is to stop the inner traversal if we encounter an AssignmentPattern. The outer traversal will still visit the AssignmentPattern, so RestElements within the right-hand side of an AssignmentPattern will be properly transformed at that time.
Woah, what's going on here?
A monorepo, muhahahahahaha. See the monorepo design doc for reasoning.
Core Packages
| Package | Version | Dependencies |
|---|---|---|
@babel/core |
 |
 |
babylon |
 |
 |
@babel/traverse |
 |
 |
@babel/generator |
 |
 |
@babel/core is the Babel compiler itself; it exposes the babel.transform method, where transformedCode = transform(src).code.
The compiler can be broken down into 3 parts:
- The parser:
babylon - The transformer[s]: All the plugins/presets
- These all use
@babel/traverseto traverse through the AST
- These all use
- The generator:
@babel/generator
The flow goes like this:
input string -> babylon parser -> AST -> transformer[s] -> AST -> @babel/generator -> output string
Check out the babel-handbook for more information on this.
Other
| Package | Version | Dependencies |
|---|---|---|
@babel/cli |
 |
 |
@babel/types |
 |
 |
@babel/polyfill |
 |
 |
@babel/runtime |
 |
 |
@babel/register |
 |
 |
@babel/template |
 |
 |
@babel/helpers |
 |
 |
@babel/code-frame |
 |
 |
@babel/cliis the CLI tool that runs@babel/coreand helps with outputting to a directory, a file, stdout and more (also includes@babel/nodecli). Check out the docs.@babel/typesis used to validate, build and change AST nodes.@babel/polyfillis literally a wrapper aroundcore-jsand regenerator-runtime. Check out the docs.@babel/runtimeis similar to the polyfill except that it doesn't modify the global scope and is to be used with@babel/plugin-transform-runtime(usually in library/plugin code). Check out the docs.@babel/registeris a way to automatically compile files with Babel on the fly by binding to Node.jsrequire. Check out the docs.@babel/templateis a helper function that allows constructing AST nodes from a string presentation of the code; this eliminates the tedium of using@babel/typesfor building AST nodes.@babel/helpersis a set of pre-made@babel/templatefunctions that are used in some Babel plugins.@babel/code-frameis a standalone package used to generate errors that print the source code and point to error locations.
Presets
After Babel 6, the default transforms were removed; if you don't specify any plugins/presets, Babel will just return the original source code.
The transformer[s] used in Babel are the independent pieces of code that transform specific things. For example: the es2015-arrow-functions transform specifically changes arrow functions into regular functions. A preset is simply an array of plugins that make it easier to run a whole a set of transforms without specifying each one manually.
| Package | Version | Dependencies | Description |
|---|---|---|---|
@babel/preset-env |
 |
 |
automatically determines plugins and polyfills you need based on your supported environments |
You can find community maintained presets on npm
Plugins
Plugins are the heart of Babel and what make it work.
You can find community plugins on npm.
Transform Plugins
There are many kinds of plugins: ones that convert ES6/ES2015 to ES5, transform to ES3, minification, JSX, flow, experimental features, and more. Check out our website for more.
Syntax Plugins
These just enable the transform plugins to be able to parse certain features (the transform plugins already include the syntax plugins so you don't need both): @babel/plugin-syntax-x. Check out our website for more.
Helpers
These are mostly for internal use in various plugins: @babel/helper-x.