# Changelog

> **Tags:**
> - [New Feature]
> - [Bug Fix]
> - [Spec Compliancy]
> - [Breaking Change]
> - [Documentation]
> - [Internal]
> - [Polish]

_Note: Gaps between patch versions are faulty/broken releases._

See [CHANGELOG - 6to5](CHANGELOG-6to5.md) for the pre-4.0.0 version changelog.

## 5.6.11

 ** **Internal**
  * Merge `es6.parameters.rest` and `es6.parameters.default` transformers. See commit [c0fd4c1f9e0b18231f585c4fa793e4cb0e01aed1](https://github.com/babel/babel/commit/c0fd4c1f9e0b18231f585c4fa793e4cb0e01aed1) for more info.

## 5.6.10

 * **Bug Fix**
  * Fix faulty internal require check.
 * **Polish**
  * Add support for trailing commas in arrow function parameter lists.

## 5.6.8

 * **Bug Fix**
  * Fix binary expressions colliding with unary expression operators in compact mode.
  * Fix node properties being set to `null` when using computed properties.

## 5.6.7

 * **Bug Fix**
  * Fix hoisting of `ForXStatement` `left` `var`s when inserting a block scoping IIFE.
 * **Polish**
  * Combine all leading computed property initialisers into the root object in loose mode.
 * **Internal**
  * Deprecate returning of replacement strings from visitor methods.

## 5.6.6

 * **Bug Fix**
  * Fix weird parser bug where `void` type annotations were being parsed as keywords causing the tokeniser to lose track of context.

## 5.6.5

 * **Bug Fix**
  * Fix nested functions causing rest parameter optimisation to not properly detect when it should deopt on a reference.
 * **Internal**
  * Update Regenerator `0.8.31`.

## 5.6.4

 * **Internal**
  * Add `ParenthesizedExpression` node type.

## 5.6.3

 * **Bug Fix**
  * Fix rest parameter array allocation loop being incorrectly aliased.

## 5.6.2

 * **Bug Fix**
  * Fix method key literals not turning into computed member expression in loose mode.
  * Elect rest parameters in spread element position as candidates instead of replacing them in place.

## 5.6.0

 * **Bug Fix**
  * Fix istanbul interop for register hook when registering for non-existence extension.
  * Fix super class constructor call differing for no constructor in derived classes.
  * Disable module import receiver when in loose mode.
  * Fix duplicate filenames when using `babel` CLI when passing multiple matching patterns.
  * Register labels as bindings to fix undeclared variable checks.
 * **Polish**
  * Remove unnecessary string binary expressions when transforming template literals.
  * Support module live bindings in arbitary positions not in Program statement position.
  * Throw error when attemping to replace a `Program` root node with another node not of type `Program`.
  * Optimise rest parameters in spread element position and allocate rest array at the earliest common ancestor of all references.
  * Generate original number representation when value was not changed.
  * Check for invalid binding identifiers when generating inferred method names.
  * Don't terminate CLI when watching files fail compilation on init.
 * **New Feature**
  * Add new plugin API.
 * **Internal**
  * Split react displayName addition into a plugin.
  * Add check for `JSXMemberExpression` to `t.isReferenced`.
  * Move `validation.undeclaredVariableCheck` transformer up.
  * Start great core-to-plugin exodus.
  * Add `BindingIdentifier` virtual type.
  * Hidden class optimisations.
  * Array allocation optimisations.
  * Update `regenerator`.
  * Update `js-tokens`.
  * Sync with upstream Acorn.

## 5.5.8

 * **Internal**
  * Remove extremely unprofessional and harsh error message for those hotlinking to `resolve-rc`.

## 5.5.7

 * **Bug Fix**
  * Push newline after decorators when doing code gen.
  * Rewriting error handling to normalise options before merging them.
  * Remove duplicate keys in `alias-keys.json` causing errors in strict mode.
  * Fix `$ babel --help` not showing optional transformers as such.
 * **New Feature**
  * Add `auxiliaryCommentBefore` and `auxiliaryCommentAfter` options.

## 5.5.6

 * **Bug Fix**
  * Fix `let` binding collision in loop head not properly replacing `AssignmentExpression`s.

## 5.5.5

 * **Bug Fix**
  * Fix `file.opts` not being set before `file.log.deprecate` was called causing a `ReferenceError` as it was checking for a property on it.

## 5.5.4

 * **Bug Fix**
  * Add back missing `shouldIgnore` check.
  * Log message on deprecated options rather than throw an error.
  * Fix name of `auxiliaryComment` option when attempting Istanbul interop in `babel/register`.

## 5.5.3

 * **Bug Fix**
  * Fix weird state bug when traversing overa  `node` `ClassProperty` instead of `path` in the `es6.classes` transformer.

## 5.5.2

 * **Bug Fix**
  * Fix `NodePath#isPure` on `Property` nodes.
  * Use cwd instead of entry file directory when working out relative directory for `babel/register`.
 * **Internal**
  * Add scary warning for those few who choose to use the WIP experimental transformers.

## 5.5.1

 * **Bug Fix**
  * Remove `ClassProperty` nodes always in the `Flow` transformer. This is fine now since class properties aren't supported in any engine that supports classes but the `es7.classProperties` transformer will need to be updated in the future to desugar to ES6 classes instead of relying on the `es6.classes` transformer from being ran.

## 5.5.0

 * **Bug Fix**
  * Allow pushing declarations to `SwitchStatement`s.
  * Fix `minification.removeDebugger` to remove `DebuggerStatement`s rather than `ExpressionStatement`s with an identifier of `debugger`.
  * Check LHS in `ForInStatement` and `ForOfStatement` for constant violations.
  * Register function `id` as a reference when naming methods to avoid collisions.
  * Support key literals when checking for the existence of `displayName` property when attempting to add it for `React.createClass`.
  * Remove `ExportDefaultSpecifier` check from `t.isDefaultSpecifier`.
  * Don't consider `JSXIdentifier` HTML tag identifiers to be references.
 * **Polish**
  * Update `minification.deadCodeElimination` transformer to remove all statements after completion statements.
  * Update `minification.deadCodeElimination` transformer to not inline single used bindings that exist in different scopes.
  * When performing Istanbul interop in `babel/register`, add the auxiliary comment `"istanbul ignore text"` to get more accurate coverage.
  * Add `--nolazy` argument to `babel-node`.
  * Add support for `cluster` forking.
  * Perform scope tracking in a single pass instead of multiple.
  * Smarten up type inferrence and resolution to support the whole array of language constructs.
  * Optimise module metadata retrieval into a single pass.
  * Ignore trailing commas when inferring newlines.
  * Rename `minification.inlineExpressions` transformer to `minification.constantFolding`.
  * Check path relative to entry file when checking to see if we're inside `node_modules` when using `babel/register`.
  * Upgrade `regenerator`.

## 5.4.7

 * **Bug Fix**
  * Don't consider `JSXAttribute` `names` to be valid `ReferencedIdentifier`s.

## 5.4.6

 * **Bug Fix**
  * Fix `spec.functionName` transformer incorrectly attempting to rename a binding that doesn't exist as it's a global.
 * **Internal**
  * Deprecate custom module formatters.

## 5.4.5

 * **Bug Fix**
  * Add `JSXIdentifier` as a valid `ReferencedIdentifier` visitor virtual type.
  * Ignore `CallExpression` `_prettyCall` when the `retainLines` option is enabled.
  * Inherit comments to new declaration node when exploding module declarations.
  * Fix `es6.tailCall` transformer failing on calls that exceed the max parameters of the function.

## 5.4.4

 * **Bug Fix**
  * Fix bug where replacing variable declarations in the head of a `for` loop would turn them into `ExpressionStatement`s.
  * Fix renaming of assignment expressions that were non-identifiers ie. patterns.
  * Force space before `class` `id` to avoid breaking named classes when using `compact` mode.
  * Add assignment pattern explosion to avoid initial duplicate nodes.
  * Ignore this and arguments when performing TCO on shadowed functions.
 * **Polish**
  * Rename `sourceMapName` option to `sourceMapTarget`. Thanks [@getify](https://github.com/getify)!
  * Better detection of completion records, ignore those in `Function`s.
  * Clarified descriptions of the options that are enabled by default.
  * Resolve `\`babel-plugin-${name}\`` plugin names **before** just checking the `name`. Thanks [@jquense](https://github.com/jquense)!
  * Update AMD module formatter to add import default remapping.

## 5.4.3

 * **Bug Fix**
  * Fix `module` being incorrectly rewritten when used as in an export declaration.
  * When performing single-reference inlining, ensure that the single reference isn't a child of the binding itself.
  * Fix a bug in `minification.deadCodeElimination` where a new binding instance was being created for local class bindings instead of just inheriting the parent one.
  * Fix bug with paren printing in `compact` and `retainLines` mode where a left paren was already printed before catching up.
 * **Internal**
  * Handle contexts for paths much better. This will ensure that the path node location info is in sync.

## 5.4.2

 * **Polish**
  * `ignore` and `only` patterns are now **very** liberal. The pattern can now exist anywhere in the path.

## 5.4.1

 * **Bug Fix**
  * Add missing `slash` dependency. Thanks [@browncolyn](https://github.com/browncolyn)!
 * **Polish**
  * Clean up `shouldIgnore` algorithm to work how you'd expect rather than being a hacky piece of shit. It now crawls the entire path, checking each section of it against the input ignore/only patterns. This means that the pattern `foo` will ignore the paths `foo/bar.js`, `bar/foo` etc.

## 5.4.0

 * **New Feature**
  * Added [function bind syntax](https://github.com/zenparsing/es-function-bind) behind stage 0. Thanks [@RReverser](https://github.com/rreverser)!
  * Added `env` option. Especially handy when using the `.babelrc`.
 * **Bug Fix**
  * Fix files not properly being ignored when `babel.transform` ignores them when using `$ babel`.
  * Fix scope tracking registering loop head bindings to their `VariableDeclaration` instead of `VariableDeclarator`.
 * **Polish**
  * Normalise path separators for souce map paths when using `$ babel`.
  * Rework `PathHoister` to ignore global references and to not deopt on reassignments to referenced bindings, instead it tries to hoist to the highest scope.
  * Added missing exponential operator inlining. Thanks [@nkt](https://github.com/nkt)!
  * Optimise `regenerator` transformer. Thanks [@benjamn](https://github.com/benjamn)!

## 5.3.3

 * **Bug Fix**
  * Fix `minification.deadCodeElimination` transformer incorrectly trying to inline import declarations.
  * Fix `minification.inlineExpression` transformer getting into an infinite loop.

## 5.3.2

 * **Bug Fix**
  * Fix patterns not being considered when hoisting variables in the `es6.blockScoping` transformer.

## 5.3.1

 * **Bug Fix**
  * Fix unique export specifiers not being cloned when exploding class and function exports,
 * **Polish**
  * Turn import remaps to sequence expressions to remove their context and improve performance.

## 5.3.0

**Speeeeeeed**

![gifs lol](https://31.media.tumblr.com/568205a0e37ae15eca510fa639589a59/tumblr_n8kw8kpcSb1sg6cg8o1_500.gif)

 * **Spec Compliancy**
  * Allow trailing param commas for methods when using the `es7.trailingCommas` transformer.
 * **Bug Fix**
  * Fix `es6.blockScoping` transformer not properly ignoring `break` in `SwitchCase`.
  * Fix lookahead context saving to avoid weird tokenizer state.
  * Explode duplicate identifiers in export/import specifiers and property shorthand to create unique objects.
  * Skip loose mode for class methods when they have decorators.
  * When removing nodes, share their comments with their siblings.
  * Properly hoist temp param declarations when doing TCO.
 * **Internal**
  * Add `--harmony_generators` flag to `$ babel-node`.
  * Internal AST traversals have been minimised **drastically**. Transformers have been grouped together which means entire tree traversals are much fewer. Visiting nodes is now also skipped if the traversal context can detect that the handler is a noop. This sames precious cycles as it avoids constructing traversal paths and creating a new traversal context. See issues [#1472](https://github.com/babel/babel/issues/1472) and [#1486](https://github.com/babel/babel/issues/1486) for related discussion.
 * **Polish**
  * Move many `utility` transformers to `minification`.

## 5.2.17

 * **Bug Fix**
  * Fix auxiliary comments not properly being attached to function declaration helpers.
  * Add `Super` node type to `ast-types` patch.
  * Ignore parameter bindings when attempting to inline them in the `minification.deadCodeElimination` transformer.
  * Correct `extensions` arguments when using the Babel CLI.

## 5.2.16

 * **Bug Fix**
  * Fix plugins being disabled when using the whitelist.
  * Fix correct function scope being passed to `nameMethod.property` when inferring the function name for class methods.
  * Fix incorrect extensions reference causing weird issues when using the Babel CLI.
  * Fix destructuring param reference replacements not inheriting from their original param.
 * **Spec Compliancy**
  * Fix order that method decorators are ran in.

## 5.2.15

 * **Bug Fix**
  * Fix initializer descriptor add attempt if it doesn't exist.

## 5.2.14

 * **Bug Fix**
  * Fix bug with initializer decorators where the descriptors weren't being defined if there was no `initializer` property.
 * **Internal**
  * Expose `retainLines` option to CLI.
  * Fix `retainLines` option not being taken into consideration when doing multiple variable declaration declarators generation.
  * Expose minified and unminified copies of dist scripts.

## 5.2.13

 * **Bug Fix**
  * Fix `ExportDeclaration`s being incorrectly removed when using the `utility.deadCodeElimination` transformer.
  * Fix position of `utility` transformers.
 * **New Feature**
  * Add built-in `esquery` support.
 * **Internal**
  * Consolidate notion of "virtual types".

## 5.2.12

 * **Polish**
  * Make UID generation based on module declarations **much** nicer.
 * **Internal**
  * Remove internal check for traversal path replacement of self. This is a pattern that **could** come up in the wild and it could lead to pretty nasty code and may lead to internal regressions as the test coverage isn't 100% :( Instead, just put it in the fast path.

## 5.2.11

 * **Internal**
  * Rename `getModuleName` option to `getModuleId`, doh.

## 5.2.10

 * **Bug Fix**
  * Fix numerous issues in `replaceWithSourceString`. Thanks [@pangratz](https://github.com/pangratz)!
 * **New Feature**
  * Add `getModuleName` option. Thanks [@jayphelps](https://github.com/jayphelps)!

## 5.2.9

 * **Bug Fix**
  * Fix `_blockHoist` transformer incorrectly sorting nodes on shitty environments that aren't spec compliant in their key order.
  * Fix broken `parse` API method reference to an undeclared import.

## 5.2.7

 * **Bug Fix**
  * Move `utility.deadCodeElimination` transformer up to avoid race conditions.
  * Fix shorthand property scope binding renaming.
 * **Polish**
  * Turn helper variable declarations into function declarations if possible.
 * **Internal**
  * Removed native inheritance support from classes.
  * Added `replaceWithSourceString` path API.
  * Split up `es3.propertyLiterals` and `es3.memberExpressionLiterals` transformers to `minfication.propertyLiterals` and `es3.memberExpressionLiterals`.

## 5.2.6

 * **Internal**
  * Fix transformer aliases being accidently set as deprecated ones.
  * Expose `Pipeline` as `TransformerPipeline` instead.

## 5.2.5

 * **Bug Fix**
  * Fix `parse` API not adding all the correct pipeline transformers.

## 5.2.4

 * **Bug Fix**
  * Fix race condition with the Node API being loaded awkwardly and not being able to initialise itself when used in the browser.
 * **Internal**
  * Expose `transform.pipeline`.

## 5.2.3

 * **Bug Fix**
  * Fix plugin containers being called with an undefined import. Thanks [@timbur](https://github.com/timbur)!
  * Allow Flow object separators to be commas. Thanks [@monsanto](https://github.com/monsanto)!
  * Add missing `Statement` and `Declaration` node aliases to flow types.

## 5.2.2

 * **Internal**
  * Allow `util.arrayify` to take arbitrary types and coerce it into an array.

## 5.2.1

 * **Bug Fix**
  * Fix regression in `node/register` that caused `node_modules` to not be ignored.

## 5.2.0

 * **Bug Fix**
  * Fix plugin strings splitting arbitrarily on `:` which caused full paths on Windows to fail as they include `:` after the drive letter.
  * Call class property `initializer`s with their target instead of their descriptor.
  * Fix `ignore` and `only` not properly working on Windows path separators. Thanks [@stagas](https://github.com/stagas)!
  * Fix `resolveRc` running on files twice causing issues. Thanks [@lukescott](https://github.com/lukescott)!
  * Fix shorthand properties not correctly being target for `isReferenced` checks. Thanks [@monsanto](https://github.com/monsanto)!
 * **Polish**
  * Allow passing an array of globs to `babel/register` `only` and `ignore` options. Thanks [@Mark-Simulacrum](https://github.com/Mark-Simulacrum)!
  * When inferring function names that collide with upper bindings, instead of doing the wrapper, instead rename them.
  * Consider constant-like variable declaration functions to always refer to themselves so TOC can be performed.
  * Process globs manually when using `$ babel` as some shells such as Windows don't explode them. Thanks [@jden](https://github.com/jden)!
  * Add alternative way to execute plugins via a closure that's called with the current Babel instance.
 * **Internal**
  * Remove multiple internal transformers in favor of directly doing things when we need to. Previously, declarations such as `_ref` that we needed to create in specific scopes were done at the very end via the `_declarations` transformer. Now, they're done and added to the scope **right** when they're needed. This gets rid of the crappy `_declarations` property on scope nodes and fixes the crappy regenerator bug where it was creating a new `BlockStatement` so the declarations were being lost.
  * Rework transformer traversal optimisation. Turns out that calling a `check` function for **every single node** in the AST is ridiculously expensive. 300,000 nodes timesed by ~30 transformers meant that it took tens of seconds to perform while it's quicker to just do the unnecessary traversal. Seems obvious in hindsight.
 * **New Feature**
  * Add `jscript` transformer that turns named function expressions into function declarations to get around [JScript's horribly broken function expression semantics](https://kangax.github.io/nfe/#jscript-bugs). Thanks [@kondi](https://github.com/kondi)!
  * Add `@@hasInstance` support to objects when using the `es6.spec.symbols` transformer.
  * Add `retainLines` option that retains the line (but not the columns!) of the input code.

## 5.1.13

 * **Polish**
  * Remove symbol check from `defineProperty` helper.

## 5.1.12

 * **Bug Fix**
  * Fix `resolveModuleSource` not being ran on `ExportAllDeclaration`s.
  * Fix `.babelrc` being resolved multiple times when using the require hook.
  * Fix parse error on spread properties in assignment position.
  * Fix `externalHelpers` option being incorrectly listed as type `string`.
 * **Internal**
  * Upgrade `core-js` to `0.9.0`.
 * **Spec Compliancy**
  * Fix object decorators not using the `initializer` pattern.
  * Remove property initializer descriptor reflection.

## 5.1.11

 * **Bug Fix**
  * Memoise and bind member expression decorators.
  * Move JSX children cleaning to opening element visitor. Fixes elements not being cleaned in certain scenarios.
  * Consider `SwitchStatement`s to be `Scopable`.
  * Fix `bluebirdCoroutines` calling `interopRequireWildcard` before it's defined.
  * Add space to `do...while` code generation.
  * Validate `super` use before `this` on `super` exit rather than entrance.
 * **Polish**
  * Add Babel name to logger.

## 5.1.10

 * **Bug Fix**
  * Remove `makePredicate` from acorn in favor of an `indexOf`.
  * Remove statements to expression explosion when inserting a block statement.
 * **Internal**
  * Remove runtime compatibility check.

## 5.1.9

 * **Bug Fix**
  * Fix class property initializers with `undefined` values not being correctly writable.
  * Fix self inferring generators incorrectly causing a stack error.
  * Fix default export specifiers not triggering AMD `module` argument inclusion.
  * Fix assignments not having their module references properly remapped.
 * **Internal**
  * Upgrade to latest `acorn`.
 * **Polish**
  * Make invalid LHS pattern error messages nicer.

## 5.1.8

 * **Bug Fix**
  * Only make parenthesized object pattern LHS illegal.

## 5.1.7

 * **Internal**
  * Add `parse` node API.

## 5.1.6

 * **Bug Fix**
  * Fix `runtime` built-in catchall not properly checking for local variables.

## 5.1.5

 * **Internal**
  * Bump `core-js` version.

## 5.1.4

 * **Polish**
  * Add missing `Reflect` methods to runtime transformer.

## 5.1.3

 * **Internal**
  * Switch entirely to vanilla regenerator.
  * Clean up and make the parsing of decorators stateless.
 * **Bug Fix**
  * Don't do TCO on generators and async functions.
  * Add missing `core-js` runtime definitions.

## 5.1.2

 * **Bug Fix**
  * Add `getIterator` and `isIterable` to `babel-runtime` build script.

## 5.1.1

 * **Bug Fix**
  * Add missing runtime symbol definitions.

## 5.1.0

 * **Bug Fix**
  * Fix super reference when using decorators.
  * Don't do array unpack optimisation when member expressions are present.
  * Add missing descriptors for undecorated class properties.
  * Don't consider `arguments` and `eval` valid function names when doing function name inferrence.
  * Fix scope tracking of constants in loop heads.
  * Parse `AwaitExpression` as a unary instead of an assignment.
  * Fix regex evaluation when attempting static evaluation.
  * Don't emit tokens when doing a lookahead.
  * Add missing `test` declaration to `utility.deadCodeElimination` transformer.
 * **Internal**
  * Upgrade `regenerator` to the latest and use my branch with the hope of eventually switching to vanilla regenerator.
  * Add support for the replacement of for loop `init`s with statements.
  * Upgrade dependencies.
 * **Polish**
  * When adding the scope IIFE when using default parameters, don't shadow the function expression, just `apply` `this` and `arguments` if necessary.
  * Use path basename as non-default import fallback.
 * **New Feature**
  * Add [trailing function comma proposal](https://github.com/jeffmo/es-trailing-function-commas). Thanks [@AluisioASG](https://github.com/AluisioASG)!
  * Add support for object literal decorators.
  * Make core-js modular when using the `runtime` transformer.

## 5.0.12

 * **Bug Fix**
  * Fix incorrect remapping of module references inside of a function id redirection container.

## 5.0.11

 * **Bug Fix**
  * Fix new `for...of` loops not properly inheriting their original loop.
 * **Internal**
  * Disable scope instance cache.
 * **Polish**
  * Allow comments in `.babelrc` JSON.

## 5.0.9

 * **Polish**
  * Use `moduleId` for UMD global name if available.
 * **Bug Fix**
  * Fix UMD global `module` variable shadowing the `amd`/`common` `module` variable.
  * Fix Flow param type annotation regression.
  * Fix function name collision `toString` wrapper. Thanks [@alawatthe](https://github.com/alawatthe)!

## 5.0.8

 * **Bug Fix**
  * Fix falsy static class properties not being writable.
  * Fix block scoping collisions not properly detecting modules and function clashes.
  * Skip `this` before `super` for derived constructors on functions.

## 5.0.7

 * **New Feature**
  * Add `--ignore` and `--only` support to the CLI.
 * **Bug Fix**
  * Remove `HOMEPATH` environment variable from home resolution in `babel/register` cache.
 * **Internal**
  * Disable WIP path resolution introducing infinite recursion in some code examples.
 * **Polish**
  * Add live binding to CommonJS default imports.

## 5.0.6

 * **Bug Fix**
  * Fix mangling of import references that collide with properties on `Object.prototype`.
  * Fix duplicate declarations incorrectly being reported for `var`.

## 5.0.5

 * **Internal**
  * Upgrade `core-js`.
 * **Bug Fix**
  * Fix arrays not being supported in `util.list`.

## 5.0.4

 * **Polish**
  * Check for top level `breakConfig` in `resolveRc`.

## 5.0.3

 * **Bug Fix**
  * Make relative location absolute before calling `resolveRc`.
 * **Internal**
  * Switch to global UID registry.
  * Add `breakConfig` option to prevent Babel from erroring when hitting that option.

## 5.0.1

 * **Bug Fix**
  * Fix duplicate declaration regression.
  * Fix not being able to call non-writable methods.

## 5.0.0

 * **New Feature**
  * Decorators based on [@wycat's](https://github.com/wycats) [stage 1 proposal](https://github.com/wycats/javascript-decorators).
  * Class property initializers based on [@jeffmo's](https://github.com/jeffmo) [stage 0 proposal](https://gist.github.com/jeffmo/054df782c05639da2adb).
  * Export extensions based on [@leebyron's](https://github.com/leebyron) [stage 1 proposal](https://github.com/leebyron/ecmascript-more-export-from).
  * UMD module formatter now supports globals.
  * Add `es3.runtime`, `optimisation.react.inlineElements` and `optimisation.react.constantElements` transformers.
  * Add stage option that replaces the experimental one.
  * Allow ES7 transformer to be enabled via `optional` instead of only via `stage`.
  * Infer string quotes to use in the code generator.
  * Consider `export { foo as default  };` to be the same as `export default foo;`.
  * Add `nonStandard` option that can be set to `false` to remove parser support for JSX and Flow.
  * Add `jsxPragma` option.
  * Automatically generate CLI options based on internal API options.
  * Add support for `.babelrc` on absolute paths.
  * Plugin API!
 * **Internal**
  * Export `options` in browser API.
  * Rewritten parser.
  * Don't block hoist when runtime transformer is enabled in system module formatter.
  * Rewritten the internal traversal and node replacement API to use "paths" that abstracts out node relationships.
 * **Polish**
  * JSX output is now more inline with the official JSX transformer.
  * Hoist block scoping IIFE - this improves memory usage and performance.
  * Better IIFE detection - references are now checked to see if they're referencing the binding we're searching for.
  * Check for import reassignments in constants transformer.
  * Make method definitions with expression bodies illegal.
  * Save register cache on tick instead of `SIGINT`.
  * Enable strict mode on babel-node eval flag.
 * **Bug Fixes**
  * Add support for live bindings. This change also increases the reliablity of export specifier renaming.
  * Add support for super update and non equals assignment expressions.
  * Rename shadow constructor binding in classes.
  * Seed next iteration bindings with previous fresh bindings when reassinging loop block scoped variables.
  * Fix new expression spread referencing the wrong constructor.
  * Call `resolveModuleSource` on dynamic imports.
  * Added `param` to list of duplicate declaration kinds.
 * **Breaking Changes**
  * The Babel playground has been removed.
  * ES7 Abstract References have been removed.
  * Experimental option has been removed in favor of a stage option.
  * Rename `returnUsedHelpers` to `metadataUsedHelpers`.

## 4.7.16

 * **Bug Fix**
  * Fix constructor spreading of typed arrays.
  * Fix break/continue/return aliasing of non-loops in block scoping transformer.

## 4.7.15

 * **Bug Fix**
  * Fix constructor spreading of collections.

## 4.7.14

 * **Bug Fix**
  * Fix constructor spreading of `Promise`.
 * **Internal**
  * Deprecate remaining playground transformers and abstract references.

## 4.7.13

 * **Bug Fix**
  * Handle comments on use strict directives.
  * Fix assignment patterns with a left side pattern.
 * **Polish**
  * Special case `this` when doing expression memoisation.

## 4.7.12

 * **Bug Fix**
  * Deprecate `playground.methodBinding`.

## 4.7.11

 * **Bug Fix**
  * Fix unicode regexes stripping their unicode flag before being passed on two `regexpu`.

## 4.7.10

 * **Internal**
  * Deprecate `playground.methodBinding` and `playground.objectGetterMemoization`.
 * **Bug Fix**
  * Fix `inputSourceMap` option. Thanks [@Rich-Harris](https://github.com/Rich-Harris)!

## 4.7.9

 * **Polish**
  * Allow `inputSourceMap` to be set to `false` to skip the source map inference.
  * Infer computed literal property names.
 * **Bug Fix**
  * Fix nested labeled for-ofs.
  * Fix block scoping `break` colliding with the parent switch case.
 * **Internal**
  * Upgrade `acorn-babel`.

## 4.7.8

 * **Bug Fix**
  * Fix computed classes not properly setting symbols.

## 4.7.7

 * **Bug Fix**
  * Fix `types` API exposure.

## 4.7.6

 * **Bug Fix**
  * Fix non-Identifier/Literal computed class methods.
 * **Polish**
  * Add a fallback if `stack` on an error is unconfigurable.
  * Hoist `esModule` module declarations to the top of the file to handle circular dependencies better.

## 4.7.5

 * **Bug Fix**
  * Don't remap` break`s to call the iterator return.
 * **Polish**
  * Use a different helper for computed classes for much nicer output. Also fixes a bug in symbols being non-enumerable so they wouldn't be set on the class.

## 4.7.4

 * **Bug Fix**
  * Rewrite named function expressions in optional async function transformers.
  * Hoist directives.
  * Remove `Number` from the list of valid `runtime` constructors.
 * **Internal**
  * `spec.typeofSymbol` transformer has been renamed to `es6.symbols`.

## 4.7.2

 * **New Feature**
  * `"both"` option for `sourceMap`.
  * Add output types to external helpers. Thanks [@neVERberleRfellerER](https://github.com/neVERberleRfellerER)!
 * **Bug Fix**
  * Fix node duplication sometimes resulting in a recursion error.
  * Ignore `break`s within cases inside `for...of`.
 * **Polish**
  * Split up variable declarations and export declarations to allow easier transformation.

## 4.7.0

 * **Bug Fix**
  * Add `alternate` to list of `STATEMENT_OR_BLOCK` keys.
  * Add support for module specifiers to `t.isReferenced`.
 * **New Feature**
  * Add `inputSourceMap` option.
 * **Polish**
  * Throw an error on different `babel` and `babel-runtime` versions.
  * Replicate module environment for `babel-node` eval.
  * Clean up classes output.
 * **Spec Compliancy**
  * Make it illegal to use a rest parameter on a setter.

## 4.6.6

 * **Bug Fix**
  * Fix incorrect method call in `utility.deadCodeElimination` transformer.
  * Fix `es6.blockScopingTDZ` transformer duplicating binding nodes.

## 4.6.5

 * **Internal**
  * `useStrict` transformer has been renamed to `strict`.

## 4.6.4

 * **Bug Fix**
  * Fix `ForOfStatement` not proplery inheriting labels.
  * When in closure mode in block scoping transformer, properly check for variable shadowing.
 * **New Feature**
  * New `utility.inlineEnvironmentVariables` and `utility.inlineExpression` transformers.

## 4.6.3

 * **Bug Fix**
  * Fix `arguments` being incorrectly aliased in arrow function rest parameter optimisation.
  * Make deoptimisation trigger safer.
 * **New Feature**
  * Flow types are now retained when blacklisting the `flow` transformer.

## 4.6.1

 * **Bug Fix**
  * Fix generators in template directory being transformed.
  * Fix exposure of `util` for plugins.

## 4.6.0

 * **New Feature**
  * Desugar sticky regexes to a new constructor expression so it can be handled by a polyfill.
 * **Spec Compliancy**
  * `for...of` now outputs in a lengthy `try...catch` this is to ensure spec compliancy in regards to iterator returns and abrupt completions. See [google/traceur-compiler#1773](https://github.com/google/traceur-compiler/issues/1773) and [babel/babel/#838](https://github.com/babel/babel/issues/838) for more information.
 * **Polish**
  * Rest parameters that are only refered to via number properties on member expressions are desugared into a direct `arguments` reference. Thanks [@neVERberleRfellerER](https://github.com/neVERberleRfellerER)!
  * `$ babel` no longer exits on syntax errors.
 * **Internal**
  * Upgrade `browserify`.
  * Upgrade `source-map`.
  * Publicly expose more internals.

## 4.5.5

 * **Polish**
  * Delete old extensions when overriding them in `babel/register`.

## 4.5.3

 * **Bug Fix**
  * Fix whitelisting logic for helper build script.

## 4.5.2

 * **New Feature**
  * `returnUsedHelpers` option and add whitelist to `buildHelpers`.
 * **Bug Fix**
  * Fix function arity on self referencing inferred named functions.
 * **Internal**
  * Bump `acorn-babel`.
  * Start converting source to ES6...

## 4.5.1

**Babel now compiles itself!**

![holy shit](http://gifsec.com/wp-content/uploads/GIF/2014/03/OMG-GIF_2.gif)

## 4.5.0

 * **New Feature**
  * Add `.babelrc` support.
 * **Bug Fix**
  * Move use strict directives to the module formatter bodies.
 * **Internal**
  * Make default `bin/babel` behaviour to ignore non-compilable files and add a `--copy-files` flag to revert to the old behaviour.

## 4.4.6

 * **Bug Fix**
  * Fix extending a class expression with no methods/only constructor. Thanks [@neVERberleRfellerER](https://github.com/neVERberleRfellerER)!
  * Allow `MemberExpression` as a valid `left` of `ForOfStatement`.
 * **Polish**
  * Throw an error when people try and transpile code with the `@jsx React.DOM` pragma as it conflicts with the custom jsx constructo method detection.
  * Crawl all comments for `@jsx` pragma.
 * **Internal**
  * Upgrade `chalk`.
  * Upgrade `core-js`.

## 4.4.5

 * **Internal**
  * Remove function self reference optimisation.

## 4.4.4

 * **Bug Fix**
  * Handle inferred function ids to be reassigned and deopt to a slower but working equivalent.
  * Don't unpack array patterns that have more elements than their right hand array expression.
 * **Polish**
  * Improve syntax highlighting in the code frame. Thanks [@lydell](https://github.com/lydell)!
 * **Internal**
  * Upgrade `acorn-babel`.

## 4.4.3

 * **Bug Fix**
  * Fix `for...of` iterator break returns being duplicated.
  * Only call `return` on the iterator if it exists.
 * **Internal**
  * Rename `selfContained` transformer to `runtime`.

## 4.4.2

 * **New Feature**
  * Add `moduleId` option for specifying a custom module id.

## 4.4.0

 * **New Feature**
  * `/*** @jsx NAMESPACE **/` comments are now honored by the `react` transformer.
  * `getModuleName` option.
  * Infer function expression names. Thanks [@RReverser](https://github.com/RReverser)!
 * **Bug Fix**
  * Add proper control flow for tail recursion optimisation.
 * **Internal**
  * Remove useless `format` options and move the `format.compact` option to `format`.
 * **Polish**
  * Newline handling of the code generator has been heavily improved.
  * Code generator now deopts whitespace if the input size is >100KB.

## 4.3.0

 * **Breaking Change**
  * Remove `commonStandard` module formatter and make it the default behaviour of all the strict module formatters.

## 4.2.1

 * **Polish**
  * Add auxiliary comment to let scoping closure flow control.

## 4.2.0

 * **Polish**
  * Use an assignment instead of a define for `__esModule` in loose mode.
 * **Internal**
  * Add error for `eval();` usage and enable strict mode for parsing.

## 4.1.0

 * **New Feature**
  * Add `BABEL_CACHE_PATH` and `BABEL_DISABLE_CACHE` environment variables.
 * **Internal**
  * Replace many internal util functions with modules. Thanks [@sindresorhus](https://github.com/sindresorhus)!

## 4.0.2

 * **Bug Fix**
  * Fix generators not properly propagating their internal declarations.
 * **Polish**
  * Update setter param length error message.
  * Use ranges on dependencies.

## 4.0.0

 * 6to5 is now known as Babel.
 * Global helpers/runtime has now been given the more descriptive name of "external helpers".
