.github/CHANGELOG-6to5.md
# 6to5 Changelog
## 3.6.6
* **Internal**
* Upgrade `core-js`.
* **Bug Fix**
* Fix temp variables not being properly pushed inside of `while` loops.
* **New Feature**
* Add `auxiliaryComment`/`--auxiliary-comment` option that prepends comments to auxiliary helpers.
## 3.6.5
* **Internal**
* Upgrade `core-js`.
* **Bug Fix**
* Fix block scoping leaking variables in IIFE mode.
* Fix tail call transformer exploding on return statements as the consequent of an if statement.
* **New Feature**
* Add `validation.react` transformer.
## 3.6.4
* **New Feature**
* Add support for flow type casts and module types.
## 3.6.3
* **Internal**
* Upgrade `acorn-6to5`.
* Use `default` property on `6to5-runtime`.
## 3.6.2
* **Polish**
* Use better iterator detection in `slicedToArray` helper and add support for it to the `selfContained` transformer.
## 3.6.1
* **Bug Fix**
* Multiline JSX string literals are now supported.
* Fix scope tracking import specifiers incorrectly.
* Fix templates incorrectly recursing into their replacements.
* **Internal**
* Type inferrence now extends to function return types.
* Upgrade `acorn-6to5`.
## 3.6.0
* **New Feature**
* Add playground class property initializers.
* **Polish**
* Completely reimplemented scope tracking, can now track types and optimise certain helpers based on it.
* Extremely fast tail recursion optimisation implementation. Thanks [@RReverser](https://github.com/RReverser)!
* **Internal**
* `kexec` has been removed as an optional dependency and must be user installed.
* Upgrade `regenerator-6to5`.
* Upgrade `acorn-6to5`.
* Upgrade `core-js`.
## 3.5.3
* Enable `es6.tailCall` transformer with the first implementation that only works with self referencing calls until we can implement nested tail calls elegantly.
## 3.5.2
* Disable `es6.tailCall` temporairly after reports of it breaking.
## 3.5.1
* **Polish**
* Allow tail calls to work across files without the runtime.
* **Internal**
* Upgrade `acorn-6to5`.
## 3.5.0
* **Bug Fix**
* Destructuring patterns as the left operator in `ForInStatement`/`ForOfStatement`.
* **Polish**
* Make default parameter IIFE invocation smarter.
* Make `__esModule` flag non-enumerable. Thanks [@daliwali](https://github.com/daliwali)!
* **Internal**
* More performance improvements.
* Parsing is now ~30% faster thanks to [marijnh/acorn@7264bc0178e7e6af7cfe02e9e0c6b26ee0e6007f](https://github.com/marijnh/acorn/commit/7264bc0178e7e6af7cfe02e9e0c6b26ee0e6007f).
* **New Feature**
* Optional `es6.blockScopingTDZ` is now completely functional and handles all edge cases.
* `super` in object literals.
* Tail call optimisation. Thanks [@RReverser](https://github.com/RReverser)!
## 3.4.1
* **Bug Fix**
* Fix conflicting `--module-ids` shorthand arg in `$ 6to5`.
* Add require hook options to cache key.
* Fix strict module formatter.
## 3.4.0
* **New Feature**
* Add `commonStandard` module formatter.
* **Bug Fix**
* Fix conflicting `--module-ids` shorthand arg in `$ 6to5`.
* **Internal**
* Lots of internal refactoring with scope tracking and traversal.
* **Polish**
* Don't return `map` in the API result if `sourceMap` was set to `"inline"`.
## 3.3.12
* **Bug Fix**
* Don't override `MemberExpression`s with `core-js` in `selfContained` if a local binding exists.
## 3.3.11
* **Bug Fix**
* Fix the require cache.
## 3.3.10
* **Internal**
* Restructure transformers so they're only ran if the AST contains nodes that they need to worry about. Improves transpilation speed significantly.
* **Bug Fix**
* Fix source maps not tracking end of node locations.
* **Spec Compliance**
* Use static super references as the home object is actually done at definition time.
* **Polish**
* Force the `es6.destructuring` transformer to be whitelisted when the `es7.objectSpread` transformer is.
* Join sibling string literals when creating JSX.
## 3.3.9
* **Bug Fix**
* Fix super inside of functions.
* Fix super constructor inheritance.
## 3.3.7
* **Bug Fix**
* Add `--use-strict` to valid node flags in `6to5-node`.
* Fix booleans not being properly stripped from the arguments in `6to5-node`.
* Force `.js` extension when writing files to directories with `6to5`.
## 3.3.5
* **Bug Fix**
* Fix block scoping inside of while loops.
* Make module name regex more conservative. Thanks [@johlrich](https://github.com/johlrich)!
* Fix block scoping of constants.
* Fix istanbul interop.
* Make JSX transforming more inline with the official transformer with spaces after non-empty last lines.
* **Polish**
* Make it illegal to export a property called `__esModule`.
## 3.3.4
* **Polish**
* Add istanbul `require` interop.
* **Bug Fix**
* Fix incorrect source map column tracking in specific scenarios.
## 3.3.3
* **Polish**
* Remap top level `this` to `undefined` instead of throwing an error.
* **Bug Fix**
* Run `selfContained` transformer over the regenerator runtime when building `6to5-runtime`.
* Fix `t.isReferenced` not properly allowing `value` nodes.
## 3.3.1
* **Bug Fix**
* Block hoist assignment pattern destructuring.
## 3.3.0
* **Bug Fix**
* Do all transforms before the regenerator transform is ran.
* **New Feature**
* Added back the 2.x optional runtime.
## 3.2.1
* **Bug Fix**
* Fix block scoping transformer rewriting breaks and continues to inner labels.
## 3.2.0
* **Bug Fix**
* Fixed scope tracking for default parameters IIFE.
* Fixed block scoped functions.
* Improved `bin/6to5` path resolution.
* **New Feature**
* You can now trigger super setters in classes. Thanks [@kruppel](https://github.com/kruppel)!
* Add `resolveSourceMap` option.
* Better support and output for block scoping loops with labels.
## 3.1.1
* **Polish**
* Drop `enumerable: false` clause from class method definitions as `enumerable` already defaults to `false`.
* **Bug Fix**
* Properly transform `XJSIdentifier` nodes referencing `this` into a `ThisExpression`.
## 3.1.0
* **Breaking Change**
* [Make class methods unenumerable](https://esdiscuss.org/topic/classes-and-enumerability#content-61).
## 3.0.16
* **Bug Fix**
* Don't ignore dynamic import specifiers.
* **Internal**
* Upgrade `regenerator-6to5`.
* **Polish**
* Use `Object.assign` in place of extends helper if it exists.
* Require individual `lodash` methods. Thanks [@stefanpenner](https://github.com/stefanpenner)!
## 3.0.15
* **Bug Fix**
* Use `debug/node` instead of `debug` to avoid browserify using the browser version that references `window`.
## 3.0.14
* **New Feature**
* Add `--optional` argument to `6to5-node`.
* **Bug Fix**
* Fix bug in `asyncToGenerator` helper where it was incorrectly calling generator iterator functions.
## 3.0.13
* **Bug Fix**
* Fix modules loose mode using `modules` instead of `es6.modules`.
## 3.0.12
* **Internal**
* Add internal debug messages.
* **Bug Fix**
* Add `noScope` option to `traverse.clearProperties`.
## 3.0.11
* **Bug Fix**
* Fix `ast-types` `RestElement` definition.
* Make `es6.forOf` loose mode more versatile and support destructuring.
## 3.0.10
* **Bug Fix**
* In `types.getIds` make sure the `declaration` inside of `ExportDeclaration` is actually a `Declaration`.
## 3.0.9
* **Bug Fix**
* Make `t.isReferenced` more powerful, actually take into consideration all contexts were identifier nodes aren't actually references.
* Don't camelcase underscores when converting a string to a valid identifier.
## 3.0.8
* **Bug Fix**
* Split up default function declaration exports due to regenerator destroying the parent export declaration.
## 3.0.7
* **Internal**
* Upgrade `core-js` to `0.4.9`.
* **Bug Fix**
* Add id to function express scope tracking.
## 3.0.6
* **Bug Fix**
* Fix block scope variable tracking stopping whenever it hits a new scope.
* Fix block scope variable tracking breaking on all block statement scopes that have a for loop parent.
## 3.0.5
* **Internal**
* More reliable default parameter scope.
## 3.0.4
* **Bug Fix**
* Remove traversal stops from block scope tracking.
## 3.0.3
* **Internal**
* Ignore options starting with `_`.
## 3.0.2
* **Internal**
* Add common plugin options to valid options list.
## 3.0.1
* **Internal**
* Downgrade `kexec` as `1.1.0` throws compilation errors.
## 3.0.0
* **Polish**
* Generated code autoindentation.
* Moved global uid registry to a scope registry resulting in nicer uids.
* `this` is now illegal in the top level scope when using the default `useStrict` transformer.
* New `asyncToGenerator` helper that's much more compact.
* Throw errors on unknown options.
* More reliable default parameter scope.
* Helpers are now compact and have no newlines.
* **Internal**
* Rewritten a lot of the internals to much simpler.
* Removed TDZ from default parameters as it was very wonky and unreliable.
* Upgrade `core-js` to `0.4.6`.
* Upgrade `acorn-6to5`.
* JSX support is now on-par with react-tools/esprima.
* Shorthand destructuring.
* **Bug Fix**
* Generators now work flawlessly with any parameter transformers.
* Optional async function transformers should not name their functions.
* Remove `unlink` event watching in `bin/6to5`.
* Fix regenerator variable declarations being incorrectly hoisted breaking scope.
* Transformers that attempted to simplify expressions that had no consequence now take into account whether it's one of the last statements/expressions in the script.
* **New Feature**
* New `selfContained` transformer.
* New `undeclaredVariableCheck` optional transformer.
* Added `--blacklist` and `--whitelist` options to `6to5-node`.
* **Breaking Change**
* Caching is now always enabled for the require hook. It also now no longer caches require resolutions.
* Bare `super();` only works inside of constructors.
* Removed the optional runtime in favor of the `selfContained` transformer.
* This shorthand has been removed from the playground.
* `6to5/polyfill` can now only be required **once**.
* **CLI**
* `--indent` option has been removed.
* `--include-regenerator` option has been removed.
* `--amd-modules-id` option has been removed, use `--module-ids` instead.
* **Options**
* `amdModuleIds` option has been removed, use `moduleIds` instead.
* `includeRegenerator` has been removed.
* `ignoreRegex` fallback has now been dropped from the require hook. `register(/foo/);`, `register({ ignoreRegex: /foo/ })` -> `register({ ignore: /foo/ })`.
* **Modules**
* Module interop now only collapses to `module.exports` when there's a **single** export that's default.
* Imports and exports are now illegal anywhere except the root level by default. Set `modules` to [loose mode](http://6to5.org/docs/usage/loose) to allow them everywhere.
* **Transformers**
* Optional fast transformer backwards compatibility support has been removed. Use [loose mode](https://6to5.org/docs/usage/loose).
* Removed the `coreAliasing` transformer in favor of `selfContained`.
* Renamed transformers:
* `specNoForInOfAssignment` -> `validation.noForInOfAssignment`
* `specSetters` -> `validation.setters`
* `specBlockScopedFunctions` -> `spec.blockScopedFunctions`
* `malletOperator` -> `playground.malletOperator`
* `methodBinding` -> `playground.methodBinding`
* `memoizationOperator` -> `playground.memoizationOperator`
* `objectGetterMemoization` -> `playground.objectGetterMemoization`
* `modules` -> `es6.modules`
* `propertyNameShorthand` -> `es6.properties.shorthand`
* `arrayComprehension` -> `es7.comprehensions`
* `generatorComprehension` -> `es7.comprehensions`
* `arrowFunctions` -> `es6.arrowFunctions`
* `classes` -> `es6.classes`
* `objectSpread` -> `es7.objectSpread`
* `exponentiationOperator` -> `es7.exponentiationOperator`
* `spread` -> `es6.spread`
* `templateLiterals` -> `es6.templateLiterals`
* `propertyMethodAssignment` -> `es6.properties.shorthand`
* `computedPropertyNames` -> `es6.properties.computed`
* `defaultParameters` -> `es6.parameters.default`
* `restParameters` -> `es6.parameters.rest`
* `destructuring` -> `es6.destructuring`
* `forOf` -> `es6.forOf`
* `unicodeRegex` -> `es6.unicodeRegex`
* `abstractReferences` -> `es7.abstractReferences`
* `constants` -> `es6.constants`
* `letScoping` -> `es6.blockScoping`
* `blockScopingTDZ` -> `es6.blockScopingTDZ`
* `generators` -> `regenerator`
* `protoToAssign` -> `spec.protoToAssign`
* `typeofSymbol` -> `spec.typeofSymbol`
* `coreAliasing` -> `selfContained`
* `undefinedToVoid` -> `spec.undefinedToVoid`
* `undeclaredVariableCheck` -> `validation.undeclaredVariableCheck`
* `specPropertyLiterals` -> `minification.propertyLiterals`
* `specMemberExpressionLiterals` -> `minification.memberExpressionLiterals`
## 2.13.7
* **Bug Fix**
* Don't realias variables that are already declared in optional `coreAliasing` transformer.
## 2.13.6
* **Bug Fix**
* Add `NewExpression` as a valid parent for parentheses insertion for `ConditionalExpression`.
## 2.13.5
* **Bug Fix**
* Allow rest parameters to be destructuring patterns.
* **Internal**
* Upgrade `kexec` to `1.1.0`.
## 2.13.4
* **Bug Fix**
* Fix ordering of destructuring and default params.
* Fix loop body scope.
* Fix `for...of` transformation not retaining it's scope.
* Add destructuring assignment support to scope tracking.
* **Polish**
* More reliable newlines for pretty printing variable declarations.
## 2.13.3
* **Internal**
* Add `coverage` and `vendor` to npmignore.
## 2.13.2
* **Bug Fix**
* Fix `super` inside of nested functions.
* **Internal**
* Move let scoping TDZ into a separate transformer until it's more solid.
* Use `process.stdin.write` instead of `console.log` in `bin` to avoid sprintfification.
## 2.13.1
* **New Feature**
* Temporal dead zone for block binding.
## 2.13.0
* **New Feature**
* Block scoped functions.
* Add `--loose all` option.
* Add playground mallet operator thanks to [@jridgewell](https://github.com/jridgewell).
* **Bug Fix**
* Rewrote large parts of the let scoping transformer. Now passes all known tests.
* **Internal**
* Even more optimisations thanks to the awesome [@gaearon](https://github.com/gaearon).
* Upgrade `acorn-6to5`.
* Fixes exponentation operator thanks to [@charliesome](https://github.com/charliesome).
* Fix flow optional parameters with types.
* Upgrade `chokidar` to `0.12.6`.
* Upgrade `commander` to `2.6.0`.
* Upgrade `core-js` to `0.4.5`.
* Upgrade `estraverse` to `1.9.1`.
* Upgrade `jshint` to `2.5.11`.
* Upgrade `regenerator` to `0.8.9`.
* Upgrade `regexpu` to `1.0.0`.
* Upgrade `source-map` to `0.1.43`.
* Upgrade `source-map-support` to `0.2.9`.
* Upgrade `browserify` to `8.1.1`.
* Upgrade `chai` to `1.10.0`.
* Upgrade `istanbul` to `0.3.5`.
* Upgrade `mocha` to `2.1.0`.
* Upgrade `uglify-js` to `2.4.16`.
## 2.12.6
* **Bug Fix**
* Add missing flow type traversal keys.
## 2.12.5
* **Internal**
* Fix incorrect `for...in` loop still causing `ember-script` issues.
## 2.12.4
* **Polish**
* Rest parameters now allocate the array before populating.
* **Internal**
* `for...in` loops have been changed to optimized `for` loops - better performance and no enumeration of prototype keys.
* Parts of the code generator have now been optimized thanks to [gaearon](https://github.com/gaearon).
## 2.12.3
* **Spec Compliance**
* Optional `typeof` transformer checks for `undefined` before passing it to the helper.
* Class methods are now named.
## 2.12.2
* **Internal**
* Exclude nodes in function parameters and catch clauses from `isReferenced` check.
## 2.12.1
* **Internal**
* Add `.jsx` to list of allowed extensions.
## 2.12.0
* **Bug Fix**
* Support non-string JSX literals.
* **New Feature**
* Loose mode for some transformers that enables non-spec behavior.
* **Internal**
* Uglify `--mangle sort` has been added to the build script, cutting minified scripts in half.
## 2.11.4
* **Internal**
* Make all dependency versions fixed.
## 2.11.3
* **Bug Fix**
* Allow a string to be passed as the `optional` option.
## 2.11.2
* **Bug Fix**
* Support esprima-style catch clause handlers.
* **Polish**
* Don't print a stacktrace for syntax errors in the CLI.
## 2.11.1
* **Bug Fix**
* Remove stray `console.log` outputting debug code.
* Remove `Dict` from `coreAliasing`.
## 2.11.0
* **Bug Fix**
* Fix unnecessary IIFE in default parameters on method calls.
* Add support for supers inside of closures.
* **New Feature**
* Add `--keep-module-id-extensions`/`keepModuleIdExtensions` option to keep extensions in module ids.
* **Polish**
* Special case single argument spread properties in `super` inside classes.
* Don't use a variable declaration for class declaration IFFE.
* Rename `inherits` helper parameters.
* `coreAliasing` transformer now aliases `Promise`, `Set`, `Map` and more. Thanks [@AluisioASG](https://github.com/AluisioASG).
## 2.10.1
* **Internal**
* Upgrade `core-js` to `0.4.4`.
* **New Feature**
* Added `--include-regenerator` option to CLI and `includeRegenerator` option to API that includes the regenerator runtime if necessary.
## 2.10.0
* **New Feature**
* Add `classesFastSuper` optional transformer that doesn't support parent getters and prototype changing.
* Add `forOfFast` transformer that speeds up `for...of` on arrays but produces more code.
* Add `--react-compat` to `bin/6to5`.
* **Spec Compliance**
* Disallow setters without a single parameter.
* Add `configurable` and `writable` to defined properties.
* Use define instead of assignment for class methods.
* **Polish**
* Fix bin name in `6to5-node --help`.
* Fix shorthand `--whitelist` name in `bin/6to5`.
* **Internal**
* Hot code optimisation of traversal etc thanks to [gaearon](https://github.com/gaearon).
## 2.9.4
* **Bug Fix**
* Support `UpdateExpression`s as `MemberExpression` objects.
## 2.9.3
* **Bug Fix**
* Remove `exportsWildcard` helper in favor of `defaults`.
## 2.9.2
* **Bug Fix**
* Pass `exports` to `exportWildcard` helper to allow for use inside the optional runtime.
## 2.9.1
* **Bug Fix**
* Fix runtime generator breaking the helper inclusion loop.
## 2.9.0
* **Internal**
* Upgrade `acorn-6to5`.
* Now supports destructuring shorthand properties.
## 2.8.1
* **Bug Fix**
* Fix computed accessors on object literals.
## 2.8.0
* **New Feature**
* Add `commonStrict`, `amdStrict` and `umdStrict` module formatters that remove `module.exports` interop assignment.
* Add `--indent` option to the 6to5 CLI.
## 2.7.4
* **Polish**
* Inherit assignments from their declaration in destructuring.
* Properly align multi-declarator variable declarations.
## 2.7.3
* **Polish**
* Indent and add newlines to `React.createElement` calls in `react` transformer.
* Remove `Object.assign` calls and replace it with an `extends` helper.
## 2.7.1
* **New Feature**
* Expose `version` on browser and node API.
* **Internal**
* Upgrade `core-js` to 0.4.1
## 2.7.0
* **Spec Compliance**
* Disallow reassignments of imports.
* **New Feature**
* `reactCompat` option to enable pre-v0.12 react components.
## 2.6.3
* **Bug Fix**
* Fix 2.6.0 regression caused by faulty export default from a source handling.
## 2.6.2
* **Bug Fix**
* Fix rest parameter keys when on functions with params.
## 2.6.1
* **Bug Fix**
* Fix rest parameter arguments key.
## 2.6.0
* **Bug Fix**
* Better handling of number literal property keys.
* Handle `NewExpression` paren insertion edge cases better.
* **Internal**
* Fix incorrect AST node `identifier("this")`.
* Better `toIdentifier` method that handles reserved words.
* Upgrade `acorn-6to5`.
* Fix exponentiation operator precedence.
* Fix line terminators after async contextual keywords.
* Add semicolons as class elements inside of a class body.
* Upgrade to `core-js` 4.0.0.
* Upgrade to `regenerator` 0.8.3.
* Fix non-loop labeled statements.
* **New Feature**
* Basic destructuring defaults
* Add `.es` to list of supported extensions.
* Add optional `typeofSymbol` transformer.
* Use a `for` loop for rest parameters instead of `Array.prototype.slice`.
* **Polish**
* Move `"use strict";` to inside module bodies instead of at the top of the file.
* Better handling of dynamic imports.
* **Spec Compliance**
* Class inheritance now has a `function` or `null` type check.
* Add `interopRequireWildcard` around wildcard imports and exports to handle non-object exports.
## 2.5.0
* Remove `noDuplicateProperties` transformer.
* Better generated UIDs based on nodes.
* Default parameters now use `arguments[i]`, conditionals and variable declarations instead of using long-form if statements.
## 2.4.10
* Upgrade `acorn-6to5`.
## 2.4.9
* Upgrade `acorn-6to5`.
* Add optional `protoToAssign` transformer.
* Fix missing properties from computed property keys.
* Make ES7 comprehensions `let` variables.
## 2.4.8
* Make `require("6to5/register")` work with browserify - [#370](https://github.com/6to5/6to5/pull/370). Thanks [@hughsk](https://github.com/hughsk)!
## 2.4.7
* Upgrade `acorn-6to5`.
## 2.4.6
* Move `coreAliasing` and `undefinedToVoid` transformers down to catch `moduleFormatter` transforms.
## 2.4.5
* Avoid printing comments if they've already been output.
## 2.4.4
* Add `module` type to browser build `<script>` handler.
* Fix some `MemberExpression` modifying incorrectly setting `property` to a `MemberExpression`.
## 2.4.3
* Upgrade `acorn-6to5`.
* Add support for `FunctionDeclaration`s in `bluebirdCoroutines` and `asyncToGenerators` transformers.
## 2.4.2
* Upgrade `acorn-6to5`.
* Better uids generated for various transformers based on parent node.
* Alias flat references in `coreAliasing` transformer.
## 2.4.1
* Better whitespace handling of parenthesized expressions due to trailing comments.
* Fix `yield` inside of comprehensions.
## 2.4.0
* Use a closure always for classes with a super.
* Always use native loops for array comprehensions.
* Allow `yield` inside of comprehensions.
* Add optional `bluebirdCoroutine` transformer.
* Add optional `asyncToGenerator` transformer.
* Move `useStrict` transformer to before `_moduleFormatter` causing `"use strict";` to always be placed the very top.
## 2.3.2
* Add parens on expressions with trailing comments.
## 2.3.1
* Add `undefinedToVoid` optional transformer.
* Use `Object.defineProperty` for computed properties.
## 2.3.0
* Upgrade `acorn-6to5`.
* Support circular references and hoist variable declarations in `system` module formatter.
* Add optional transformers, including a new `coreAliasing` transformer that aliases native ES6 static properties to their `core-js` equivalent.
## 2.2.0
* Make `system` module formatter modules anonymous by default.
* Fix duplicate comments being output, breaking code.
## 2.1.0
* Add `cache` option to register hook.
* Update `core-js`.
* Fix starting newline not being added on case statements.
* Fix destructuring `VariableDeclaration`s not inside `BlockStatement`s and `Program`.
## 2.0.4
* Avoid being greedy when destructuring array iterables.
## 2.0.3
* Hoist function declarations in system module formatter for circular references.
* Hoist default function declarations in umd and amd module formatters for circular references.
## 2.0.2
* Inherit comments in `for-of` transformer.
* Remove `interopRequire` from `system` module formatter.
## 2.0.1
* Remap `UpdateExpression` module export binding.
* Fix automatic closure on `PrivateDeclaration` in classes.
## 2.0.0
* Make string literal generation only escapes unicode that it has to.
* Internal code generation format options have been exposed.
* Change playground method binding operator from `:` to `#` removing ambiguous syntax with terns.
* Fix rest parameters in async and generator functions.
* Export/import declarations replace by the modules transformer now inherit comments.
* Added playground flag to `6to5-node`.
* `6to5-node` now behaves the same as `node`.
* `6to5-node` now uses `kexec` to become the forked process to correctly propagate signals on unix.
* Constants are now block scoped.
* Exposed ast transformer.
* Merged `commonInterop` and `common` module formatters.
* Fix generator comprehensions not inheriting `arguments`, `this` etc.
* Object and class mutator shorthand are now enumerable.
* Remove regenerator `Generator has already finished` error which isn't spec-compliant.
* Expose internal `spec` transformers that nicen up code output.
* Add export variable declaration default initializers.
* Propagate export declaration reassignments.
* Add initializer default to block scoped variable declarations within a loop.
* Flow type support.
* Make async/await contextual keywords.
* Allow `yield`ing of non-objects.
* Class declarations now lack an IIFE.
* Support falsy and `null` super classes.
* Add support for experimental abstract references `private` declarations.
* Leave out IIFE for class declarations.
* Switched to [core-js](https://github.com/zloirock/core-js) from [es6-symbol](https://github.com/medikoo/es6-symbol) and [es6-shim](https://github.com/paulmillr/es6-shim/) for built-in polyfill.
* `amd` and `umd` module formatters now behave the same as `common` with `interopRequire`.
* Micro-optimizations to boost performance by 200%.
* Rename module formatter methods `import` to `importDeclaration` and `export` to `exportDeclaration`.
* Support multiple declarators in export variable declarations.
* Freeze tagged template literal object.
* Remove inlined `regenerator` fork.
* Remove `ParenthesizedExpression`.
* Rename `object-spread` helper to `object-without-properties`.
* Rename `class-props` helper to `prototype-properties`.
* Rename `extends` helper to `inherits`.
* Completely rewritten `system` module formatter.
## 1.15.0
* Don't alias `GeneratorFunction` and check the name which causes minifiers to remove the name and throw an error later on when we check if it's set.
## 1.14.18
* Fix files only containg comments not being output.
* Fix duplicate comments on property key shorthands.
## 1.14.17
* Add default initializer to let variables within loop bodies.
* Fix excessive `break` replacement inside of switches in let scoping.
## 1.14.16
* Add object getter memos and this shorthand to playground.
* Fix while loops in let scoping.
* Upgrade `acorn-6to5`.
## 1.14.14
* Fix template literals escaping.
## 1.14.13
* Fix let scoping of `while` loops.
* Make class methods enumerable.
## 1.14.12
* Fix duplicate dynamic expressions in call spread.
## 1.14.10
* Fix let scoping unnecessary override.
## 1.14.6
* Avoid ensuring a block on non-array node replacements.
## 1.14.5
* Upgrade `acorn-6to5`.
* Fix JSON recursion error for unknown code generator node types.
* Ensure that a statement is a block on block/statement types when replacing them with multiple nodes.
## 1.14.4
* Merge pretzel maps and method binding.
## 1.14.3
* Add playground pretzel maps.
## 1.14.2
* Fix `commonInterop` default export handling.
* Fix keyworded property key identifiers being turned into computed property key literals.
## 1.14.1
* Inherit comments from `ClassDeclaration`.
## 1.14.0
* Add [playground](https://6to5.github.io/playground.html).
## 1.13.13
* Fix `--debug` in `bin/6to5-node`. Thanks [@timoxley](https://github.com/timoxley).
## 1.13.12
* Ignore `XJSEmptyExpression`s in `react` transformer output.
## 1.13.11
* Fix `util.regexify` on falsy values.
* Fix `_aliasFunction` with rest parameters.
* Export as `module.exports` instead of `exports.default` if there are no other `ExportDeclaration`s in `commonInterop` module formatter.
* Add `system` module formatter. Thanks [@douglasduteil](https://github.com/douglasduteil).
## 1.13.10
* Add support for `AssignmentExpression` destructuring outside of `ExpressionStatement`.
## 1.13.9
* Fix `VirtualPropertyExpression` visitor keys.
## 1.13.8
* Only use a single reference in abstract references.
## 1.13.7
* Upgrade `acorn-6to5`.
* Add experimental exponentiation operator support.
## 1.13.6
* Fix experimental object spread/rest helper.
## 1.13.5
* Upgrade `acorn-6to5`.
* Add experimental support for object spread/rest.
* Change `arguments` to array to an additional helper method.
## 1.13.4
* Fix single spread element returning itself.
## 1.13.3
* Upgrade `acorn-6to5`.
* Add experimental support for abstract references.
## 1.13.2
* Optimize `Array.from` usage by adding a helper method.
* Upgrade `acorn-6to5`.
## 1.13.1
* Fix constructor spread optimisation. Thanks [@zloirock](https://github.com/zloirock).
## 1.13.0
* Put experimental ES7 features behind a flag `--experimental` and `experimental` option.
* Constructor spread performance increase. Thanks [@RReverser](https://github.com/RReverser).
* Use `self` instead of `window` in the optional 6to5 runtime. Thanks [@RReverser](https://github.com/RReverser).
## 1.12.26
* Support computed property destructuring.
## 1.12.25
* Update `acorn-6to5`, `ast-types`, `es6-shim`, `chokidar`, `estraverse` and `private`.
## 1.12.24
* Collect references that haven't been declared in scope.
## 1.12.23
* Fix generator function export hoisting.
## 1.12.22
* Update `fs-readdir-recursive` and `chokidar`.
* Support array destructuring on iterables.
* Make amd module id optional. Thanks [@webpro](https://github.com/webpro).
## 1.12.21
* Fix unnecessary let scoping replacement.
* Add `commonInterop` module formatter. Thanks [@Naddiseo](https://github.com/Naddiseo).
* Fix `return` outside of function body bug. Thanks [@brentburg](https://github.com/brentburg).
* Add more flexible option types.
## 1.12.20
* Append `sourceMappingURL` when using `bin/6to5` and output sourcemaps.
## 1.12.19
* Add `comments` option and `--remove-comments` flag. Thanks [@webpro](htps://github.com/webpro).
* Embed `regenerator`.
## 1.12.18
* Use `global` reference instead of `window`.
## 1.12.17
* Add `moduleName`, `sourceRoot` and `filenameRelative` options. Thanks [@darvelo](https://github.com/darvelo).
* Traversal optimisations.
## 1.12.16
* Fix comments not being retained from `MethodDefinition` in classes.
* Add temporal dead zone in default parameters.
## 1.12.15
* Update `acorn-6to5`.
## 1.12.14
* Fix duplicate let scoping in functions.
* Make JSX whitespace more React-compliant.
* Add `_memberExpressionKeywords` transformer that turns keyword identifiers to computed literals.
* Upgrade `regenerator-6to5`.
## 1.12.13
* Support duplicate constants within different block scopes.
* Fix for-head duplication testing and replacement.
* Support `raw` property on tagged template literals.
## 1.12.12
* Make scope tracker more reliable to handle all edge cases.
## 1.12.11
* Block scope classes.
* Fix generation of integer `Literal`s in `MemberExpression`.
## 1.12.10
* Fix let scoping var hoisting.
## 1.12.9
* Escape unicode characters when generating string `Literal`s.
* Fix semicolons being output for statements in `ExportDeclaration`.
* Fix `WithStatement` missing parenthesis.
## 1.12.8
* Temporarily forbid `AssignmentExpression` destructuring outside of `ExpressionStatement`.
## 1.12.7
* Update to latest `acorn-6to5`.
## 1.12.6
* Update to latest `acorn-6to5`.
## 1.12.5
* Fix excessive whitespace trimming resulting in innaccurate sourcemap line.
## 1.12.4
* Add `doc` folder for documentation.
## 1.12.3
* Support generator comprehensions.
* Use `Array.from` instead of `Array.prototype.slice` in spread transformer.
* Support spread in `NewExpression`s.
## 1.12.2
* Upgrade `matcha` to `0.6.0` and `browserify` to `6.3.2`.
* Add own `trimRight` helper instead of relying on the string instance method.
* Support JSX spreads that aren't the first.
## 1.12.1
* Fix `this` and `arguments` mapping in the `_aliasFunctions` transformer.
## 1.12.0
* Combine `jsx` and `react` transformers to `react`.
* Update `react` syntax output to React v0.12.
## 1.11.15
* Fix JSX literal whitespace generation.
## 1.11.14
* Avoid using a switch for let-scoping continue and break statements and use an if statement instead.
* Remove excess whitespace and newlines from JSX literals.
## 1.11.13
* Update regenerator-6to5
* Add support for most escodegen formatting options