ECMAScript compatibility tracking issue

[teppeis]

This is a tracking issue for failed compat-table tests with links to each input, output, error and related issue (updated with v20200426).

These items are filtered out as "won't fix":

• new Function("features"): hard to implement
• Proxy: impossible to implement
• subclassing for built-ins: impossible to implement (except for Promise)
• typed arrays: won't fix? (IE11 has typed array classes but doesn't have Array methods, should be fixed?), #3064
• shared memory and atomics (ES2017): impossible to implement

---

ES2015

Optimisation

proper tail calls (tail call optimisation)

• direct recursion (in/out)
• mutual recursion (in/out)

Syntax

default function parameters

• arguments object interaction (in/out)

rest parameters

• function 'length' property (in/out)
• arguments object interaction (in/out)

spread syntax for iterable objects

• with sparse arrays, in array literals (in/out)
• with astral plane strings, in function calls (in/out)
• with astral plane strings, in array literals (in/out)
• spreading non-iterables is a runtime error (in/out)

object literal extensions

• computed accessors (compile error)

for..of loops

• with astral plane strings (in/out)
• iterator closing, break (in/out): https://github.com/google/closure-compiler/issues/2958
• iterator closing, throw (in/out): https://github.com/google/closure-compiler/issues/2958

octal and binary literals

• octal supported by Number() (in/out)
• binary supported by Number() (in/out)

template literals

• toString conversion (in/out)
• tagged template literals (in/out)
• passed array is frozen (in/out)
• TemplateStrings call site caching (in/out)
• TemplateStrings permanent caching (in/out)

RegExp "y" and "u" flags

• "y" flag (in/out)
• "y" flag, lastIndex (in/out)
• "u" flag (in/out)
• "u" flag, non-BMP Unicode characters (in/out)
• "u" flag, Unicode code point escapes (in/out)
• "u" flag, case folding (in/out)

destructuring, declarations

• with astral plane strings (in/out)
• iterator closing (in/out): https://github.com/google/closure-compiler/issues/2958

destructuring, assignment

• with astral plane strings (in/out)
• iterator closing (in/out): https://github.com/google/closure-compiler/issues/2958

destructuring, parameters

• with astral plane strings (in/out)
• iterator closing (in/out): https://github.com/google/closure-compiler/issues/2958

Unicode code point escapes

• in identifiers (in/out)

new.target

• in constructors (compile error)
• assignment is an early error (compile error)

Bindings

const

• temporal dead zone (in/out)
• temporal dead zone (strict mode) (in/out)

let

• temporal dead zone (in/out)
• temporal dead zone (strict mode) (in/out)

Functions

arrow functions

• no "prototype" property (in/out)
• lexical "super" binding in constructors (compile error)
• lexical "new.target" binding (compile error)

class

• class name is lexically scoped (compile error)
• computed names, temporal dead zone (in/out)
• methods aren't enumerable (in/out)
• implicit strict mode (in/out)
• constructor requires new (in/out): https://github.com/google/closure-compiler/issues/2919
• extends null (in/out)
• new.target (compile error)

super

• constructor calls use correct "new.target" binding (compile error)

generators

• can't use "this" with new (in/out)
• %GeneratorPrototype% (in/out)
• %GeneratorPrototype% prototype chain (in/out)
• %GeneratorPrototype%.constructor (in/out)
• yield *, astral plane strings (in/out)
• yield * on non-iterables is a runtime error (in/out)
• yield *, iterator closing via throw() (in/out)
• shorthand generators can't be constructors (compile error)

Built ins

Map

• constructor requires new (in/out): https://github.com/google/closure-compiler/issues/2919
• iterator closing (in/out): https://github.com/google/closure-compiler/issues/2958
• Map iterator prototype chain (in/out)
• Map[Symbol.species] (in/out)

Set

• constructor requires new (in/out): https://github.com/google/closure-compiler/issues/2919
• iterator closing (in/out): https://github.com/google/closure-compiler/issues/2958
• Set iterator prototype chain (in/out)
• Set[Symbol.species] (in/out)

WeakMap

• constructor requires new (in/out): https://github.com/google/closure-compiler/issues/2919
• iterator closing (in/out): https://github.com/google/closure-compiler/issues/2958
• WeakMap.prototype isn't an instance (in/out)

WeakSet

• constructor requires new (in/out): https://github.com/google/closure-compiler/issues/2919
• iterator closing (in/out): https://github.com/google/closure-compiler/issues/2958

Reflect

• Reflect.ownKeys, symbol keys (in/out)
• Reflect.construct sets new.target meta-property (compile error)
• Reflect.construct, Promise subclassing (in/out)

Promise

• Promise.prototype isn't an instance (in/out)
• Promise[Symbol.species] (in/out)

Symbol

• typeof support (in/out)
• symbol keys are hidden to pre-ES6 code (in/out)
• symbols inherit from Symbol.prototype (in/out)
• cannot coerce to string or number (in/out)
• can convert with String() (in/out)
• Object(symbol) (in/out)
• JSON.stringify ignores symbol primitives (in/out)
• JSON.stringify ignores symbol objects (in/out)
• global symbol registry (in/out)

well-known symbols

• Symbol.hasInstance (in/out)
• Symbol.isConcatSpreadable (in/out)
• Symbol.iterator, arguments object (in/out)
• Symbol.species, existence (in/out)
• Symbol.species, Array.prototype.concat (in/out)
• Symbol.species, Array.prototype.filter (in/out)
• Symbol.species, Array.prototype.map (in/out)
• Symbol.species, Array.prototype.slice (in/out)
• Symbol.species, Array.prototype.splice (in/out)
• Symbol.species, RegExp.prototype[Symbol.split] (in/out)
• Symbol.species, Promise.prototype.then (in/out)
• Symbol.replace (in/out)
• Symbol.search (in/out)
• Symbol.split (in/out)
• Symbol.match (in/out)
• Symbol.match, RegExp constructor (in/out)
• Symbol.match, String.prototype.startsWith (in/out)
• Symbol.match, String.prototype.endsWith (in/out)
• Symbol.match, String.prototype.includes (in/out)
• Symbol.toPrimitive (in/out)
• Symbol.toStringTag (in/out)
• Symbol.toStringTag affects existing built-ins (in/out)
• Symbol.toStringTag, new built-ins (in/out)
• Symbol.toStringTag, misc. built-ins (in/out)
• Symbol.unscopables (compile error)

Built in extensions

Object static methods

• Object.getOwnPropertySymbols (in/out)

function "name" property

• function statements (in/out)
• function expressions (in/out)
• new Function (in/out)
• bound functions (in/out)
• variables (function) (in/out)
• object methods (function) (in/out)
• accessor properties (in/out)
• shorthand methods (in/out)
• symbol-keyed methods (in/out)
• class statements (in/out)
• class expressions (in/out)
• variables (class) (in/out)
• object methods (class) (in/out)
• class prototype methods (in/out)
• class static methods (in/out)
• isn't writable, is configurable (in/out)

String static methods

• String.raw (in/out): https://github.com/google/closure-compiler/issues/3136

String.prototype methods

• String.prototype.normalize (in/out)
• String.prototype[Symbol.iterator] (in/out)
• String iterator prototype chain (in/out)

RegExp.prototype properties

• RegExp.prototype.flags (in/out)
• RegExp.prototype[Symbol.match] (in/out)
• RegExp.prototype[Symbol.replace] (in/out)
• RegExp.prototype[Symbol.split] (in/out)
• RegExp.prototype[Symbol.search] (in/out)
• RegExp[Symbol.species] (in/out)

Array static methods

• Array.from, iterator closing (in/out)
• Array[Symbol.species] (in/out)

Array.prototype methods

• Array iterator prototype chain (in/out)
• Array.prototype[Symbol.unscopables] (in/out)

Math methods

• Math.fround (in/out)
• Math.cbrt (in/out)

Date.prototype[Symbol.toPrimitive]

• Date.prototype[Symbol.toPrimitive] (in/out)

Misc

prototype of bound functions

• basic functions (in/out)
• generator functions (in/out)
• arrow functions (in/out)
• classes (in/out)
• subclasses (in/out)

Object static methods accept primitives

• Object.getPrototypeOf (in/out)
• Object.getOwnPropertyDescriptor (in/out)
• Object.getOwnPropertyNames (in/out)
• Object.seal (in/out)
• Object.freeze (in/out)
• Object.preventExtensions (in/out)
• Object.isSealed (in/out)
• Object.isFrozen (in/out)
• Object.isExtensible (in/out)
• Object.keys (in/out)

own property order

• Object.getOwnPropertyNames (in/out)
• Reflect.ownKeys, string key order (in/out)
• Reflect.ownKeys, symbol key order (in/out)

Updated identifier syntax

• var ⸯ; (in/out)
• var 𐋀; (compile error)

miscellaneous

• duplicate property names in strict mode (compile error)
• accessors aren't constructors (in/out)
• RegExp constructor can alter flags (in/out)
• RegExp.prototype.toString generic and uses "flags" property (in/out)
• built-in prototypes are not instances (in/out)
• function 'length' is configurable (in/out)

Annex b

non-strict function semantics

• hoisted block-level function declaration (in/out)
• labeled function statements (compile error)
• function statements in if-statement clauses (compile error)

proto in object literals

• basic support (in/out)
• multiple proto is an error (in/out)
• not a computed property (in/out)
• not a shorthand property (in/out)
• not a shorthand method (in/out)

Object.prototype.proto

• absent from Object.create(null) (in/out)
• present in hasOwnProperty() (in/out)
• correct property descriptor (in/out)
• present in Object.getOwnPropertyNames() (in/out)

RegExp.prototype.compile

• returns this (in/out)

ES2016+

2016 features

Array.prototype.includes

• %TypedArray%.prototype.includes (in/out)

2016 misc

generator functions can't be used with "new"

• generator functions can't be used with "new" (in/out)

strict fn w/ non-strict non-simple params is error

• strict fn w/ non-strict non-simple params is error (in/out)

2017 features

Object static methods

• Object.getOwnPropertyDescriptors doesn't provide undefined descriptors (in/out)

async functions

• no line break between async and function (in/out)
• no "prototype" property (in/out)
• cannot await in parameters (compile error)
• correct prototype chain (in/out)
• async function prototype, Symbol.toStringTag (in/out)
• async function constructor (in/out)

2017 misc

RegExp "u" flag, case folding

• RegExp "u" flag, case folding (in/out)

2017 annex b

Object.prototype getter/setter methods

• defineGetter, ToObject(this) (in/out)
• defineSetter, ToObject(this) (in/out)
• lookupGetter, ToObject(this) (in/out)
• lookupGetter, data properties can shadow accessors (in/out)
• lookupSetter, ToObject(this) (in/out)
• lookupSetter, data properties can shadow accessors (in/out)

assignments allowed in for-in head in non-strict mode

• assignments allowed in for-in head in non-strict mode (compile error)

2018 features

object rest/spread properties

• object rest properties (in/out): https://github.com/google/closure-compiler/issues/3139
• object spread properties (in/out): https://github.com/google/closure-compiler/issues/3139

s (dotAll) flag for regular expressions

• s (dotAll) flag for regular expressions (compile error)

RegExp named capture groups

• RegExp named capture groups (compile error)

RegExp Lookbehind Assertions

• RegExp Lookbehind Assertions (compile error)

RegExp Unicode Property Escapes

• RegExp Unicode Property Escapes (compile error)

2018 misc

Proxy "ownKeys" handler, duplicate keys for non-extensible targets

• Proxy "ownKeys" handler, duplicate keys for non-extensible targets (in/out)

template literal revision

• template literal revision (in/out)

2019 features

Array.prototype.{flat, flatMap}

• flat and flatMap in Array.prototype[@@unscopables] (in/out)

2019 misc

Symbol.prototype.description

• undefined description (in/out)

Function.prototype.toString revision

• functions created with the Function constructor (in/out)
• arrows (in/out)
• [native code] (in/out)
• class expression with implicit constructor (in/out)
• class expression with explicit constructor (in/out)
• unicode escape sequences in identifiers (in/out)
• methods and computed property names (in/out)

Well-formed JSON.stringify

• Well-formed JSON.stringify (in/out)

2020 features

BigInt

• basic functionality (compile error)
• constructor (compile error)
• BigInt.asUintN (in/out)
• BigInt.asIntN (in/out)
• BigInt64Array (compile error)
• BigUint64Array (compile error)
• DataView.prototype.getBigInt64 (compile error)
• DataView.prototype.getBigUint64 (compile error)

optional chaining operator (?.)

• optional property access (compile error)
• optional bracket access (compile error)
• optional method call (compile error)
• optional function call (compile error)

[teppeis]

Fixed in v20180506:

• super: is statically bound (in/out): #2873
• generators: correct "this" binding (in/out): #2873
• Map: -0 key converts to +0 (in/out): #2905
• Set: -0 key converts to +0 (in/out): #2905

[teppeis]

Fixed in v20180610:

• Array.from map function, array-like objects: #2911
• Array.from map function, generator instances: #2911
• Array.from map function, generic iterables: #2911
• Array.from map function, instances of iterables: #2911

compat-table: https://github.com/kangax/compat-table/pull/1316

[teppeis]

Fixed in v20180716:

• no changes

Fixed in v20180805:

• ES2018: Asynchronous Iterators / async generators: https://github.com/google/closure-compiler/commit/02f59ac8e8f1b139960b699bcd0aacae23fffdb6

compat-table: https://github.com/kangax/compat-table/pull/1334

[teppeis]

Fixed in v20180910 (https://github.com/kangax/compat-table/pull/1350)

• ES2015: Classes / extends expressions #2997
• ES2018: Asynchronous Iterators / for-await-of loops

Fixed in v20181008 (https://github.com/kangax/compat-table/pull/1359)

• ES2018: Template literal revision

[teppeis]

In v20181028, "object rest properties" (in/out) is degraded: #3139

[teppeis]

Fixed in v20181125 (https://github.com/kangax/compat-table/pull/1379)

• Math.fround is supported (requiring Float32Array) #3141

[teppeis]

v20190121

• degrade: ES2018 object spread properties https://github.com/google/closure-compiler/issues/3139

v20190215

• support: ES2019 optional catch binding
• support: ES2019 JSON superset
• improve: ES6 Math.hypot https://github.com/google/closure-compiler/pull/2117
• improve: ES6 Array.prototype.copyWithin #2902

https://github.com/kangax/compat-table/pull/1422

[lauraharker]

Created internal issue http://b/125851124 for bookkeeping

[teppeis]

v20190301: https://github.com/kangax/compat-table/pull/1429

• Support: ES2019 Symbol.prototype.description
• Support: ES2019 Array.prototype.{flat, flatMap}
• Support: ES6 new Symbol() throws
• Degrade: ES6 Reflect.ownKeys with symbols

v20190325: https://github.com/kangax/compat-table/pull/1443

• Support: Object.fromEntries
• Support: String.prototype.trimStart / trimEnd depending on String.prototype.trimLeft / Right (they are not implemented yet)

[teppeis]

v20190709: https://github.com/kangax/compat-table/pull/1482

• Support: ES2019 string trimming

v20191027: https://github.com/kangax/compat-table/pull/1531

• Support: ES2020 Promise.allSettled

[l1bbcsg]

What's the status on Nullish Coalescing Operator (foo ?? bar) and Optional Chaining (foo?.bar)?

[teppeis]

@l1bbcsg Not yet. https://github.com/google/closure-compiler/blob/6ca3b62990064488074a1a8931b9e8dc39b148b3/src/com/google/javascript/jscomp/parsing/parser/FeatureSet.java#L187-L188

[l1bbcsg]

I saw that one actually 😀 I mean, is there a plan on how to deal with both of those? Can we expect them or are there some obstacles? Neither of the operators is mentioned in this issue and Optional Chaining is also nowhere to be found either in code or other issues, gives an impression they're not tracked at all.

[brad4d]

@vxlydia is actively working on implementing ?? now, and will likely start working on optional chaining in January.

At the moment I expect we'll have them available in ES_NEXT in 2020 Q1, and I hope to have ES_2020 LanguageMode available in Q2.

[teppeis]

v20200101: https://github.com/kangax/compat-table/pull/1565

• support: ES2020 globalThis
• support: ES2020 String.prototype.matchAll

[brad4d]

FYI, I've just created a "supported features" page here https://github.com/google/closure-compiler/wiki/Supported-features

Hopefully it's not too out of sync with this issue.

I added a link to this issue at the top of it.

Thanks @teppeis for your continuing efforts to maintain this information.

[teppeis]

v20200112: https://github.com/kangax/compat-table/pull/1571

• support: ES6 Classes: computed static accessor properties (https://github.com/google/closure-compiler/commit/bd0832c68248137447fee25f6b8eb37fd7f0b753)

[teppeis]

v20200315: https://github.com/kangax/compat-table/pull/1618

• Support: ES2020 nullish coalescing operator (??)
• Tagged template is broken #3589